I think thralls are decent but hard to play, they are objective holders that can do good damage the only problem they got is that they can engage mostly under high tide, because they die too easily so they absolutly have to strike first.
Last game i played 3x10 Thrall ,one unit used as objective holder, other 2 flanking with soulscrier.
They were really usefull.
Eels are probably better in most situation, but for objective capture the only thing that counts are bodies so 10 "cheap" models, that in the right situation can do some decent damage, are welcome.