Any other advantages to single detachment Ynnari?

I don't think that reasoning holds water, personally. What would make the USF formation different from the dozen other formations that ARE allowed?

I think it's a fluff reason: the Ulthwe Strike Force represents Eldar loyal to Craftworld Ulthwe, who have little to no interest in serving Ynnead.

But individual squads have no problem? I could see that, I still think it's structural because it's FOC based rather than being a formation, but that could definitely be the justification for making it a malleable detachment rather merely an open ended formation (0+ on each option min 1)

I checked on this, and I do see what you're saying amorrowlyday. It does refer to the USF as a detachment instead of a formation, so that might be it. In that case, that's silly, and it should be a formation instead, haha.