Saves could get very easy to make in a fast way. I think it is assumed that the spellcasters would be spending many of their bonus feats and ability ups to increase their casting attribute. That and the uber monster that have abilities with saves aproching the triple digit mark. It's an extension of tossing spells at a dragon, even at 15th-20th level. He's probably going to make his save.
Like alot of other things in the ELH, which I personally like, the ONE unbalancing thing is not the spells, feats, skills, or anything else. It is the lack of structure. Any two wizards would be of similar power even at 20th level, even if one was a specialist with a lot of meta magic, and one was a general wizard with alot of item creation feats. As another example, with epic you could have one wizard with an INT 10 higher than another, one that has all his metamagics practicaly free, and another with almost twice the spell slots as either of the other two. Turning all of the abilities into feats lists that can be taken at any order can lead to extreams that are harder to find at lower levels because of the class structure. Imagine if a monk did not focus on movement or AC, but, with the right feats, could become an outsider at 4th or 5th level, when DR 5/+1 really means alot.