The biggest drawback I guess with RC-cars would be the limited functions... This is why I use them mostly to test out conversions of plastic model kits (Trumpeter Kilo, Seawork, maybe a Revell VIIc, etc) without having to purchase expensive micro-sized RC-equipment. A dynamic diver could easily achieved by just using 2 RC-cars (of 2-channel each and different frequencies also) that way you'd have for example:

Car 1 (27mhz)

-Forward/Reverse
-Rudder

Car 2 (49 mhZ)

-Dive planes (I think they are easier to use than thrusters but it's your choice)
-Extra Channel

With the extra channel you can even command a pump for a ballast tank (makes it a static diver in this case)! Or even add functionnal torpedoes!