I love RPGs, but man, they usually fail at having a reasonable difficulty curve. Sometimes you'll just be plowing through the game wrecking everything that crosses your path, then bam, you hit That One Dungeon.
The most recent RPG I've played, Ar Tonelico, suffers from a mild case of this. In general, the game is extremely easy. You just gain levels and equipment and spells way too fast for the game to be challenging, and I didn't even bother trying to get all the fanciest equipment in the game. However, there is one "dungeon" near the very end of the game that deserves a special mention: the Plasma Bell. It is the second to last dungeon in the game, but it has the toughest enemies in the game, and they are pretty high above everything else in the game. The reason this place deserves special mention is because it is the first place in the game where someone in my party was killed. The place still isn't really difficult because you can always run from combat with 100% success, and with care and patience even the enemies there can be killed.
I played through the game a second time (actually, this run is still in progress, but I kind of stopped playing) going only with the initial equipment, no enhancing the equipment, and no installs to power up song magic. That actually made things tough, and it was a much more enjoyable experience. After playing for a few hours, the random encounters became dangerous, and the bosses took forever to kill, and it was really enjoyable. Phase 1 (of 3) was still manageable, and the end boss of that phase took forever because it could summon minions (which also took forever to kill) and you could generally only hurt it with magic when the minions were in the way. Then phase 2 really kicked my ass. There was one really annoying segment where I was without a magic user, and due to the rather poor way things were set up, without a magic user you can't run from battles, and you're also pretty weak without a magic user. So I was fighting inescapable battles against tough enemies, and near the end of this stretch I went down from 3 party members to 2, and I had to fight several battles in a row against rather tough enemies that could kill me in a few hits. The whole segment was frustrating beyond belief. At the end of phase 3, there were 3 boss fights in a row; the first one was pretty easy, the second one was tough, and the third one... well, on my first attempt, the third boss one-shotted my entire front line, leaving my magic user exposed. Eventually I overcame that. Then came phase 3, the final phase. Out of curiosity, I was first going to try for the bad ending, because you can do it sooner. Except I had to fight a boss that was really fast and could heal a ton of her HP very frequently. So nope, I had to go through things the long way. Soon I got to the plasma bell, and man that place was total hell. I believe that's still where I am.
So yeah, the game has a ton of different subsystems for making your party stronger, and yeah, it breaks the game beyond all form of challenge if you use them. One of the really amazing things I noticed going from the normal playthrough to the challenge playthrough was that one mechanic, charged attacks, was completely flipped on its head. Charged attacks are attacks by the enemy that target your magic user and give you advance notice, where you can then use your front line fighters to defend against the attacks, and then you can launch a super duper counter attack. In a normal playthrough, these attacks are relatively dangerous because they are really damaging to your fighters when they defend. What changed in my challenge playthrough was that these attacks instead became my means of survival. For one, the counter attacks were usually a really good form of offense, but beyond that there was a really huge hole in the mechanic. A lot of bosses and some tough enemies were capable of one-shotting my party members, but when you defend against charged attacks, the worst that can happen is your party members are left with 1hp, which again doesn't matter when lots of attacks will one-shot you anyways. Another mechanic is that special abilities used 10%, 20%, or 30% of a party member's HP, and they could be used even if you don't actually have enough HP, so again I just spammed away.
(I kind of just used this as an excuse to talk about the game)
tl;dr RPGs don't have much difficulty balance usually.