FYI my last two boats had scratch built pistons which fitted inside the central ballast tank that was operated by a pump in pump out system. A smaller piston does not give the pressure or imbalance problems the larger ones do but has all the advantages. As soon as the surface is breached, turn on the pump and empty the ballast. I haven't seen many boats with this config (none actually) but it is a very good option. The piston give fantasic fine control to get neutral buoyancy, and is triggered by the failsafe if the signal dies.
I can't actually think of a downside.