Krypta is nice in some situations, particularly on fun maps that have shadow beasts for the priestesses to tame, but Agrela's healers are overall much more useful, PARTICULARLY in the Northern expansion.
First of all, they get Turn Undead which is freaking amazing, secondly they are amazing on any map that has rats. Why? Because rat plague is a major bitch, and healers WILL AUTOMATICALLY CURE PLAGUED INDIVIDUALS. For FREE.
Furthermore, the healing herbs are considerably more useful than the poisonous herbs the cultists plant. Rangers don't generally have that much gold on hand, and healing potions will get used many, many times. Poisoning only happens once, and is completely outclassed by just upgrading the rogue's guild to level 2, something you'll want to do anyways. Plus, rangers can stockpile more than 5 potions at a time by collecting herbs.
That said, I do love the cultists of Fervus... I mean, c'mon. They're lovable little lunatics. As for Lunord/Helia, I personally kinda prefer Lunord's adepts. Sure, the solarii are beasts in combat compared to the adepts, but the adepts are great for large towns that are a bit more spread out. There are only so many guardhouses you can build before things get particularly pricy, and the guards themselves can't be relied upon to defeat anything more dangerous than a large rat. The adept's speed and their multiple teleport abilities let them get into (and, if needed, out of) dangerous situations like that and just keep order.
Generally I'll have plenty of other heavy hitters out there to smack the big boys around, but for keeping the peace and cleaning up the occasional troll/ratman raid, adepts are great.