Even though Planescape was my favorite campaign setting up until Eberron, I do not have much to say about the flavor of yugoloths, and even less about gehreleths.
To me yugoloths are just another kind of fiendish subset that seems to have been designed to fill in an alignment-spoke on the Great Wheel which, surprise surprise, is not enough to get me interested.
Neither does a high concept of, "greedy, unreliable mercenaries", for that matter.
Or a huge list of resistances.
From what I can gather from the article, yugoloths seem to tread the middle ground between Lawful and Chaotic, which makes sense because they are Neutral Evil, and...that is about it. There is nothing about them appearance-wise that even suggests they differ much from devils or demons:
|Nycaloth and mezzoloth|
|A devil, obviously.|
Speaking of the Blood War, I am not even sure what hooks to use with them without having a high-level, full-blown war campaign. I guess a wizard could summon one, only to have it escape? Maybe one gets trapped on the natural world/Material Plane and becomes a warlord? These could work, but they also sound just as feasible for a variety of other summoned/extraplanar things.
It is nice that they are parsing down spell-like abilities, as I have always felt that massive lists of spell-like abilities were unnecessary, but I think all of those resistances are also unnecessary since none of them seem to fit a theme. Like, I get that a lot of devils have fire resistance what with them being in Baator/Nine Hells, but why does every yugoloth have resistance against acid, cold, and lightning, too?
When it comes to individual yugoloths I do not have any issues for the most part; but I would keep the mezzoloth at six legs because of its insect-like appearance, and while I guess the nycaloth could stand to lose a pair, why not have both? A four-armed nycaloth could just be a more powerful incarnation.
Ultimately they will probably all end up looking very similar to their 2nd and 3rd Edition counterparts, which is a shame because as with many of the other Wandering Monsters articles there is a chance to be creative and give them a stronger identity (and, possibly, adventure hooks). I could see them bring rolled into demons given that many of them seem similar, but I do not think it is necessary or will do much to differentiate them. If nothing else, stick to calling them yugoloths instead of daemons.
(Oh, I almost forgot: either call the unique yugoloth Anthraxus or something else entirely. Changing it to Phraxus just to be politically correct is silly.)
And then there are gehreleths.