Yugoloths suck, but they don't suck quite as bad as demodands/gehreleths.
In my own game, the line between dark fae and demons is already blurred (fae look humanoid, and demons look monstrous), and one-off unique yugoloths like Shemeska fit right into that. Strict and distinct categorizations may be aesthetically pleasing to some, but I prefer my evil extradimensional monsters to be mysterious.
Pathfinder has done some awesome things with Fiends, including their version of Daemons, Demodands, And other fiend races. Daemons (they don't call them Yugoloths) are the ultimate nilists, they want to see everything dead and destroyed, including themselves after everything else is dead.
Demodands became the servants and worshippers of the Abyssal Titans qho rebelled against the Gods, and eventually created the Demodands.
Honestly the only interesting Yugoloths in MTOFs is the Marrenloths and the Oinioloths, because you can see uses for them beyond blood war mercanies.
I could see Githyanki hiring a Marrenloth to captain their flying ships for example. I also find it interesting that the Marrenloths are almost pacifists by nature, which is odd for fiends.
I can see Gods and Priests of Gods like Talona hiring Oinoloths to spread and cure plagues on her behalf.
Succubi as an independant race feels like a wasted opportunity, they could a released different types of Succubi. Also unlike devils and demons Succubi breed true among themselves, so Succubi can increase their numbers without mortal souls, aren't locked out of the Mortal Plane like Devils and perhaps other types of fiends.
A small group of Succubi can effectly create colonies of Succubi.