Too much nesting.
I'm with Pierre WRT the fact that lo quantifiers are different from all the others, actually. I don't think it's a good idea.
It doesn't much matter to me which one makes more sense; it is both an inconsistency and a change to past usage.
We already have other ways of saying "three groups of five"; "ci lo mumei broda" does the right thing, does it not?
Given that, why have the inconsistency? "Three out of some five-some of doctors" may not mean much, but it means something, and it'll be a lot less confusing that way, nevermind the possiblity of invalidating past usage.
Here's an example usage with lo quantifiers being the same as all others:
doi lo re no prenu ko fendi ko lo mumei gi'e ci lo mu prenu cu bevri lo mudri gi'e lo re drata cu kajde fi lo rokci
Not amazingly important, but certainly not completely without use.
-Robin