[Ygorl]

For their cost, the research required to summon them, and the single-nation limitation, they'd better be superior to black servants! Umbrals are nice, no doubt, but I've never seen them make or break a game. Keep in mind that you only get one per casting, so to amass a horde you need a lot of mage time (and at least in the early age you either need expensive mages or 4x that many cheap mages). While they're powerful, and a great value if you just look at gem cost, I don't think they remotely overpower the nation. I don't even think they dramatically out-cool other things Agartha can do - for the same gem cost, I can get five magma children, which aren't too shabby on their own, or a big earth elemental that can trample all over unfriendly chaff-mobs. The only cheap EA Agarthan mages that don't have a useful summon, actually, are the WE guys. I guess they can research or something.
If you restrict your attention to things you can summon with death gems, even, I don't think Umbrals are that obvious a choice. Say you've got five Oracles of the Dead and a bunch of death gems. You could summon a pile of Longdead Horsemen and with the rest of the oracles do a bunch of research or forge some things or preach a lot or lead some armies, or else for the same gem cost summon five Umbrals and do nothing else. It's not clear to me that the second choice is always or even usually better.