There are lots of problems with the Epic Spellcasting system, many of which I've hit head-on in my game, which has reached 23rd level (a fact you'd already know if you've been reading our story hour).
1). Spell are Too Expensive. Try recreating a fireball, lightning bolt and horrid wilting as cast by a 20th level caster using the Epic Spellcasting system. Prepare to be suprised after you do. Epic level spellcasting costs a huge amount in research (even 20th level characters rarely have 230,000 g.p. just lying around) and experience, let alone the compromises necessary.
2.) Too dull. Most epic level spells don't FEEL Epic, IMHO. A spell that does 20d6 damage? Sure it does it at 2000 feet, but so what? That's worth all that effort? I've got a cheaper solution...an epic archer. Super Mage Armor. Yawn. Peripety. Whooppee. Let me Go? Big deal.
3.)Too much metagame preparation required. To be an epic spellcaster, you need to max out certain skills, and keep them maxed out all the way untli you are an epic caster. Spellcraft suddenly becomes a big deal (where it may have been ignored by that cleric, druid or sorceror) as does Knowledge(X), which may also have been a low priority. Asking a character to wait until 30th level to get a high-enough spellcraft to cast a spell that's not much more powerful than his 9th level spells were at 17th level is not a good design choice.
4.)Too much work. Creating an epic level spell feels like making a Champions character, and I don't mean that as a compliment. It requires number crunching and metagame computations, all to try and manipulate the system to generate a result. It forces the players to min-max to a degree I find silly, reminding me of silly character build choices from GURPS, for example, where anosmia was a popular character quirk.
All in all, I think they tried to be so safe with Epic Spellcasting, that they drained all the flavor and fun out of it. The only player I have who has Epic spellcasting abilities simply took 10th level slots for metamagic purposes.