I have been building my subs with the setup ,2 seperate servos and at deferent oposing throw rates ,no long linkages once ballanced the sub is perfectly controlable with precition at scope depth and I dont use a leveler although I am getting lazy in my old age and might instal one from choppers use.
Some say it does not work but then I have heard that before.
In the springs and caves that I run in I like quick responding subs in order to enjoy the rock formations and tight areas so if I had to control 2 controls to do one thing would be just not fun.
Take the SB-1 the makers built her with the above system and no leveler and it holds a great mast depth ,if it had one.
True is even for my Robbe Type XXI and 5ft Alfa though the very small dive planes respond very slowly, so size and placement is critical.
Also when (flying) under water the closer to a nuetral boyancy point the better for overall control reaction that way your not fighting the oposing forces of rise and dive.
Just my experiance.