Say I want to make an Epic Lightning Bolt. Let's use the Energy seed. We could cast a regular lightning bolt as a DC 19 spellcraft check, if we're willing to wait a minute to do it. Make it take a standard action, and we're throwing a normal lighting bolt with a higher save DC at DC 39 spellcraft. But that's an awful amount of damage, I want to be able to take out half of someone's health if they fail the save.
Take a 21st level fighter with Con of +6. That's 10+5.5*20+6*21, or 236 hp. We want to reliably do 118 damage, then. That's 118/3.5, or 34d6. That requires adding 24d6 to our spell. The spellcraft DC is now 87. A 21st level character has 24 ranks in Spellcraft and maybe a +10 Int mod. This spell is not castable without working at your Spellcraft modifier.
All I've really shown is that epic spells are unfeasible to use in general combat. More math to talk about buff spells and other non-combat uses would be nice, but I have to go.