Doug McCrae said:It would be like a book devoted to flinds. I can't see anything conceptually interesting about yugoloths that separates them from other fiends. Nycaloths are just another demon. Mezzoloths are another weird fiend, visually similiar to ice devils. And ultroloths are anodyne supervillains, inappropriate in a fantasy game and lacking any kind of interesting schtick. In terms of concept, they're at best rehashes.
Rounding out the alignments was a reasonable idea, but the execution was totally bereft of originality. If you want a neutral evil force of Blood War mercs and power brokers you'd be better off taking some of the interesting demons/devils and making them neutral evil.
Fey, savage humanoids, githyanki, angels, slaad, modrons, elementals, lycanthropes, constructs and oozes should get books before yugoloths do.
How much about yugoloths do you know before saying that, may I ask?
Yugoloths work nicely in the scheme of things D&D-wise. You have to think of yugoloths as "THE Fiends" of the D&D Game when comparing them to Devils and Demons.