It wasn't so much the caster imbalance as the difference between the most powerful heroes and the weakest heroes. Due to stats like BaB and Defense you're going to easily have high level characters taking on armies and just not caring because hey your attack bonus is so high you're hitting them on a 1 and your defenses are so high they can't hit you. It's also not necessarily player fragility, there's nothing I hate more than Rocket Tag in something as swingy as a d20 system it's more ther difference between a commoner and a legendary hero. But I suppose other than that there'd really be no reason to bother with Epic 6.
Doesn't happen in FC unless you use static NPCs or screwup building them. Don't compare PC classes, compare PCs to NPCs.
The Peasant NPC at TL 1 has a +0 to attack and a first level Soldier has +1 Defense. at TL 20 the peasant has a +12 to hit and a Soldier 20 has a Defense bonus of +12. At the top of the scale the peasant actually has a slightly easier chance hitting the hero then at first level, everything else being equal.
At first level, the Soldier has a BAB of +1, and the same Peasant has a Defense of +1. At level 20 the Soldier's BAB is +20 and the Peasant's defense is +12. So the Soldier's gotten a decent amount better at killing peasants over the course of 20 levels.
Of course, why does it matter? Why is the Soldier killing peasants anyway? More importantly, if the GM wants peasants that are harder to hit, why did he leave their defense at III?
A few other problems I've got whilst I'm here and have some attention. How does damage scale compared to health? Looking at the health of enemies versus the rate at which player damage increases it seems that damage scales much slower. Also what am I supposed to do about the fact that as levels increase, crits begin becoming more and more lethal because wounds NEVER increase.
Not so much. Attribute bonuses are lower. Magic items work differently and aren't as important. The thing with Specials is they're supposed to be hard to kill. Which is why Fencers, club users, or anyone else who can convert to subdual or stress for free has an advantage there.
As for criticals... they don't get more much more dangerous. Of course, critical hits are one of the ways of removing a special NPC without dealing with the metric assload of vitality they have. I recommend grades of Tough, sometimes applied retroactively to make sure the fight isn't over anti-climatically. As for as the PCs go, they don't get exposed to that many. The GM decides when his threats become critical hits and only Special NPCs can score them anyway.