I don't know that I'll be able to properly playtest the 4e epic adventures for ZEITGEIST, and might just have to eyeball them and go by my experience running a few epic games in the past. For final encounters I'll try to wrangle some players to test them, but I doubt I can manage it for every fight.
In 2011 I ran a mini-adventure with 30th level PCs trying to stop a plane-hopping Lolth from conquering Rio de Janeiro. Action denial was the rule of the day, with the all-powerful evil goddess being knocked prone repeatedly and given a -7 penalty to her attack rolls by the bard.
Not to mention the revenant swordmage who found a way to keep fighting as a ghost while at negative hit points, teleporting Lolth back to the 'kill zone' if she ever left and tried to attack someone else. Said swordmage was bolstered by a warlord with the ability to prevent any ally he could see from dying as long as he was alive. So Lolth couldn't actually reach the warlord because she kept getting knocked prone, and if she did get there and attack him she had a massive penalty, and even then she'd get teleported back into some Zone of Death the PCs had set up, then tripped again.
Kinda anticlimactic.
So expect epic scenarios, with some generous slathering of "ways not to be gimped" for each enemy. I might just steal 5e's "legendary resistances" and say that certain enemies can just automatically ignore conditions a certain number of times. It feels a little cheap to completely negate the PCs' powers, but man, epic-level 4e is not balanced as published.