Viable or No? I've used them only a handful of times, their models are quick + easy to assemble and don't look too bad (imo) despite being from another age. Against undead they're beastly for the WS reduction and they hit zombies AUTOMATICALLY b/c they go to WS 0, which is nice. 142 pts of yhetees (with a greyback) = 10x str5 magical I4 attacks to a flank that can really bolster your ogre horde's combat. Yeah, I know that's 2x MFC, but they're I4 str5 attacks + stomps and if they get under the protection of trollguts, they become durable, to boot. Just be sure and not pair them up against flame banner folk! With any of the other buffs, they become that much more beastly and in a unit of 6-7, they're not too expensive, can disrupt and are fast moving + fast attacking.
I've only used them a couple times, but they've been worth the points each time and tend to get ignored in lieu of the ogre horde or sabertusk that's about to mess up enemy horde movement/charge a warmachine. I suppose in most situations MFC are better, but take what your enemy doesn't expect and they'll be facing a new set of challenges. Trying to pit of shades me to death (or any of the Init test "kill you" spells people use to kill ogres)? I4, baby! Coupled with a lore of death SM who gets the -3 to LD spell, that's reliable auto hit str5 attacks on units that are effected by fear. They're under appreciated, but not crappy and...they're YHETEES!! I know people will say MFC are a better investment, but give them a try, if you haven't!