I don't see Bacuna as a full back at all.
If you keep the ball well the main things you want from your fullbacks are the ability to get beyond the winger and make things happen and the pace to get back into position if it breaks down. Where him, Lowton and Bennett have struggled is that whilst they're decent in both of those regards they're not defensively strong enough to hold out in a siege, which is pretty much what we been faced with for the last few years.