Thanks for your replies so far!
engine:
My main misgivings are:
The complexity of even a very straightforward epic group, particularly with so many options for immediate actions, which play havoc with play-by-post.
Agreed. I have thought about this issue as well, but there doesn't seem an easy fix other than asking the players to keep the power-choices that are Immediate Interrupts and Reactions to a minium. There will still be the need for retroactive changes to posts, but I hope everyone can agree that those should be avoided if possible to keep the combat flowing.
engine:
The potential ease with which epic options can result in monstrously powerful synergies (constant stuns, impossible-to-hit characters, etc.), both in individual characters and the whole party. I'd hope there would still be challenge.
I hope so as well, but I'm rather optimistic when it comes to this point. I have several inspirational sources of how to handle epic enemies and encounters to help me deal with this potential problem. In the end I guess we will have to wait and see how things turn out, but I'm up for the challenge!