I'm guessing you never did the original Diadem, OP?

That version was absolutely awful, with groups simply grinding on the same island, for the entirety of the instance timer.

I'm not sure how much better Diadem 2.0 was, since I only entered it once, and it just looked like they simply picked Diadem and threw FATEs in it + plus a rare spawn raid boss.

Personally, I prefer Eureka, mainly because it's one piece of land, instead of a bunch of islands sprinkled over a zone.