For those in NYC... are non-metallic floor boxes allowed in apartments? I haven't seen anything in the NYC amendments to the NEC that prohibits their use.
Are they going to be able to survive construction? Lathers are very unforgiving when installing rebar and wire mesh. No one is going to be gentle around your work, especially when it's on the floor.
While it might be permitted, should you do it? Also floors usually provide a fire rating, between units, metal might be required for that floor assembly.