its true from my experience,
we've had this problem for years in our (me and NWG_Jonas) map Battle for Middle Earth, when the (9) nazgul die in battle they are supposed to revive after 8 minutes (or so). but when like 7+ of them die in one battle not all of them are revived for some reason..
But in general we have like 50ish heros in total and triggered revive does work when a few heros die.