To go back to the original post for a second - the wiki says that the damage from demolition ammo is supposed to be (X+2)d2, where X is the dice rolled by the original weapon. In this case, the '6' in the chaingun's original 1d6 is completely ignored and the damage is based off the '1' instead. That said, 3d2 is still a fair bit better than 1d6 - it deals about 30% more damage on average.
As for assembly balance in general, the ones I always find myself making are:
Tactical Boots, Armour, Shotgun: Everybodys' favourites. Moving fast and shooting often is obviously way more powerful than almost anything else.
Micro Launcher: A perfect compliment to the average no-EE no-Wiz shotgun run. Is vastly inferior to the missile launcher, but is easier to get and what else are you spending tech mods on?
More situational:
Chainsword: A must-have for certain challenges and malicious blades runs.
Gatling Gun, Hyperblaster: I don't do rapidfire runs often, but these are both nice when I do.
Grappling Boots: Melee-only. Tech mods would otherwise be worthless to a melee character, so I'll sometimes take 50% less knockback. Melee characters are also the only people with enough inventory space to consider boots-swapping, so you can take them off if you want to run fast or be propelled by an explosion.
Speedloader Pistol: The simplest pistol to get, I generally don't get Wiz in my AoMr runs.
Others worth mentioning:
Literally anything that requires a special mod: Isn't worth mentioning. Aside from nano mods, I never think about assemblies when I use a special mod.
Tactcial Rocket Launcher and Focused Double Shotgun: Both intrigue me, but I never go for Wizkid in a shottyman run.
Assault Rifle: I wish I could get this to work, but you need TH 2 and Wiz 1 and by then it just isn't worth it uuugh
Piercing Blade: I made this for the achievement once. It says '+2 die if you put the power mod on first' - does this mean 6d6 or 4d8? Either way, a single bulk mod gets you to 5d6 and armour really couldn't be any less of a problem to a melee character who can deal upwards of 40 damage in one shot without even trying.
Elephant Gun: I'm a bit surprised to hear this one get attention too. Its main upside over the double shotgun seems to be ammo efficiency (more damage per shell AND less damage falloff) but this doesn't matter at all to a shotgunner. The double shotgun puts out more damage per turn, the tactical shotgun fires faster (and has even less falloff), does this have a niche between them?
Fireproof Armour: I always forget this exists! I generally try to agility-mod my red armour, but most endgame enemies deal fire damage (and only revenants will ever melee you, really) so we should all try to remember to make this just before Hell.