If for some reason you want/need to go head down and youre using your BC, wings, etc for buoyancy control, you simply end up in a head down position. If youre using your drysuit, you may end up heading feet first to the surface (possibly without fins).
If for some reason you want/need to go head up and youre using your BC, wings, etc for buoyancy control, you simply end up in a head up position. If youre using your drysuit, that latex seal around you neck may burp sending you feet first to the bottom (dont laugh, this is what finally convinced me it might be worth the effort to learn to manage both).
Either way, you end up with a loss of buoyancy control that you were trying to avoid in the first place. If you end up at the surface from a loss of buoyancy using your BC, you can vent and descend again. If you end up at the surface with your legs acting as lift bags and your fins no where in site, thats how your going to stay. Hopefully your buddy or someone comes along and helps get you back on your feet.
If your suit fails (rip, tare, blown seal etc.) and youre using it for buoyancy control, you have two problems; suit flood and loss of buoyancy
task overload? Perhaps. If your suit fails and your using your BC, youre cold, wet and bitter but at least your still in control.
In truth, its probably best to be well versed in both modes of buoyancy control.