Epics are cheap omnibus books of one title. So if their is a big crossover, the epics don't collect it very well.
Epics are cheap in price, so it's good, but with a paper a little less good than some recent omnibus or tpb.
The good point of Epics is that they collect all chapters of one title !
But I like small omni that collects a full run, a crossover, or stories that are linked.
Whit Epics you don't really have themed books.
and for the x-men reading order:
https://community.cbr.com/showthread...f-X-Men-Comics