Do you have the:
Dom 3DB at:
http://koti.welho.com/ehalttun/Dom3/ ?
If you look up illithids there you will see they are a 50 gold unit. So while you make think you are fighting "only" 30 illithids, that is 1500 GOLD in units.
From what little I know of R'yleh I think the Greater Otherness and Vastness are SUMMONED or Void Gate or some such thing monsters.
The Mindless undead are probably doing better than you think against such a swarm of high cost units.