So, I've been thinking about MAc versus other traits as I play.
First of all, I think that the two rapidfire traits (MAc and MCa) SHOULD be more powerful than the other master traits. For starters, they don't include any defensive traits and they block TaN. Also, I think it makes sense that rapidfire weapons should be stronger than shotguns or pistols. They are in real life, aren't they? And I think it works out to a nice balance--rapidfire weapons are straightforward and (relatively) easy to win with. Shotguns are great in the early game, then gradually become less and less powerful versus the enemies you face... but they have a strategic value that can make up for this if you get the right traits and play to the weapons' strengths. Pistols are the hardest ranged weapons, but you can do very cool stuff with them if you pour the right traits into them.
So I don't think you necessarily need to directly compare rapidfire traits with other weapon types. I'm mostly looking at MAc versus MCa.
First of all, I think most people agree that Int is way better than MCa, especially now that enemy AI has improved. They should probably switch the two--making Int a master trait that includes both levels 1 and 2 of Int, and Cateye a 2-level advanced trait that gives you one more square of visibility per level. Most of the time MCa isn't worth the trouble, especially since it blocks off other traits.
Second, I still think that people tend to overpraise MAc and overlook the advantages of Int. MAc is very straightforward--just shoot, shoot, shoot. And it does work quite well.
But I'm finding Int can be great, too, if I adjust my strategy. I make sure to have weapons that can use all three kinds of ammo, allowing me to scavenge whatever I find on the level. Stockpiling ammo is a huge priority. The Wall is a must do (fortunately Int2 makes it a cakewalk). I usually have around 15 of my slots devoted to ammo by the time I reach Hellgate--not a problem, since I almost never have to worry about getting overwhelmed by enemies up to that point, and knowing where all the powerups are makes medkits much less essential. When I get to Phobos Hell, I just gradually use up my ammo, increasing my healing and escape items as I go. I make sure to have weapons allowing me to take advantage of all three ammo types, so I can scavenge stuff as I go.
One great thing about Int/MCa is its versatility. It goes well with any ranged weapon--in fact, if I happen to find a Blaster, Beretta, or Trigun, I can switch to SoG and give myself a powerful, ammo-efficient alternative to rapidfire weapons. And of course, if ammo does run out, Intuition is very good for avoiding enemies as well. I just make sure to pick up a level or two of HR along the way.
Game Hunter, I think you made a very good point, about MCa being better on easier difficulty levels, while MAc is better on hard difficulty levels. On the standard game, I would say that MCa is definitely better on ITYTD. You get less experience and plasma rifles don't appear until very late, meaning you won't be able to take full advantage of MAc until you've almost won. Meanwhile, an Int run will have you seeing enemies' locations from very early on, and you'll probably be able to kill every enemy you meet with ammo to spare. HNTR and HMP favor MCa and MAc, respectively, but only by a small amount and you'll get different mileage based on your playstyle, and the RNG could push things either way.
On UV, the balance takes a huge shift in favor of MAc. You will get your plasma rifle and Ammochain very early, along with 2 agility mods. This means you are guaranteed to be well-equipped to take on anything from early on in the game. Chances are you'll get at least 1 or 2 each of a power and tech mod, as well, putting your damage output higher than MCa. Meanwhile, there are massive hordes of nasty enemies, often backed by Arch-Viles. No matter how much ammo you have, it will go away fast if you try to kill everything. And a lot of them are blocking the exit, making running away difficult. You can still win, but you have to play smart and take advantage of everything the RNG gives you. And it's a LOT harder than MAc, which doesn't get much more difficult no matter how many enemies you run into.
N! shifts the balance back a little bit. You get more ammo, and ammo-producing enemies respawn, which takes a little bit of the pressure off. But the extra damage output you get from Finesse and Whizkid still pushes MAc ahead of MCa.
About challenge modes: In general I think they're pretty balanced--both have challenges they are good and bad for. For example, AoLT and AoP favor ammochain builds, because inventory space is at a premium. AoI, on the other hand, usually gives you an abundance of inventory space, and makes you more dependent on powerups (which increases Int's value). AoH is possibly the most heavily weighted against Ammochain. Half as many levels means it will take you longer to get Ammochain--on HNTR I think you could kill every enemy and still possibly not get it. Plus, you are no longer guaranteed any agility mods. Half as many hell levels also means that it's a lot easier to make it to the end without ammo running out, which works in favor of other builds.
AoMC is an interesting one. At first I thought Ammochain seemed like the best way to go, since EE is useless. But since hitting enemies before they hit you is so critically important, this makes Int a LOT much more useful. Note that you'll also be using less than half as much ammo as a regular game, since you do almost double damage and never miss. Another thing is that AoMC really takes the edge off of Arch-Viles, which usually drain your ammo and are just the bane of any non-ammochain run. On AoMC, though, it becomes much easier to gib corpses so they can't be revived, and their fire attack is no more powerful than the normal game. I've had really good results just ignoring master traits and getting Int, HR, and Fin.
Anyway, like I said, I think it's a good thing that some builds are better in some challenges than others, as long as it balances out. There's only one challenge that needs to be changed IMO and that's Ao100. That challenge is more heavily weighted towards Ammochain than any other build. You don't have a guaranteed chainsaw, which makes melee builds impractical. Shotguns? Forget it. And unless you find a blaster or nuclear plasma rifle fairly early, you're going to have a really hard time with rapidfire weapons or pistols. The fact that you have to go through 100 levels is monotonous enough, but having your options limited on what build to use makes this even worse.
So the question is, what to be done?
OK, here are my suggestions. This is what I think would help balance out MAc:
-Get rid of the double agility mods in chained court, like Game Hunter said. Instead give us random mods, or one mod and something useful but not gamebreaking like plasteel boots.
-Lower the plasma rifle's accuracy to +0, and raise the chaingun to +2. This would make an unmodded chaingun just good enough that if you had SoB/Triggerhappy and no EE, it would at least outclass the combat shotgun as a general purpose weapon. And the plasma rifle would really suffer without double agility mods (you might even want triple). Combined with the lack of guaranteed agility mods, this means you could actually have a tough time later on.
-Give us a little more ammo in the late levels. Maybe make it so that bullet stacks can give random amounts up to 50, and shotgun shell stacks can give up to 20. And make ammo spawn slightly more often on UV. Give us just enough that having to depend on found ammo still adds to the difficulty, but isn't quite so crippling in the late game.
-Ammochain blocks HR. This might be a little excessive, but if occurred to me as one more way to nerf MAc a little. It would force you to deal with one of MAc's big weaknesses: increased vulnerability. No HR and no TaN means killing everything before it gets you is even more important--and no HR makes this harder to do. Fewer agility mods would make this even more of a potential handicap.
-In Ao100 games, how about instead of neverending hell levels, how about if we had 5-level cycles? Every 5 levels it would switch between Phobos Base and Phobos Hell levels. The base levels would still have just as many nasties like Arch-Viles, Revenants, and Mancubi, but they would also generate a lot more former humans/sergeants/captains. You would still need to stockpile ammo during the Phobos base levels, but it would be much more doable.
I think these changes would balance it out pretty well. On easy difficulty levels, Int would be better, but taking advantage of Int would depend on being experienced enough to plan ahead and take advantage of all the different things the game gives you. So I think it would even out pretty well for a beginner. HMP and N! would be about evenly split between the two. MAc would still have a slight advantage in UV, but small enough that either build is quite viable. And Ao100 would be compatible with pretty much any build.
So what do you think? More experienced players, please tell me what you think.