I think it's widely accepted that the rules at Epic levels are just, on the whole, worse. However that doesn't mean that Epic games can't still be fun.
What are they like? That depends on a lot of stuff. It's not even optimisation level, really, it's more... I dunno, awareness level? Take an unoptimised Fighter and she can still go and use her L21+ wealth to pay an NPC to cast a spell which can alone break most naive plots. An actual Wizard can be so much worse, but the point is that it's about how you approach problems.
You can just play a L21+ version of a regular dungeon crawl, where instead of goblins you fight expert drow swordsmen and instead of ogres, high-level dragons who hover above pits of acid, and where everything's trapped and warded out the wazoo. However you all have to pretend that you're basically in a lower-level game just with bigger numbers, because as soon as someone starts saying "huh, I'm a ninja, one of the suckiest classes in the game, and I've still just trivialised your entire concept of a dungeon because I can walk through walls" it becomes a sort of arms-race of breaking and shoring up campaign plots which rapidly explodes into the sort of mind-bending complexity that an Epic game can be and often is.
Maybe Google Tippyverse - the whole concept is kind of dumb (a game played by the rules strictly as written; that doesn't make sense because the RAW just flat-out don't work) but it gives you an idea of how unlike classic high fantasy the D&D rules actually are at the higher levels.
So yeah it can be fun, but managing expectations and figuring out what kind of game you want is even more important in Epic levels than it is normally, and it's kind of important at the best of times. That almost doesn't even have anything to do with optimisation as per se (or at least not as one might think of it), which is a kind of whole other headache on top of that.