One of the disadvantages is that under the current technical level, magnetic fluid propulsion technology is costly and slow.
The second disadvantage is that this technology requires a large generator, which increases the weight of the ship or submarine, and the power system that drives the generator, such as a direct drive traditional propeller, is faster than MHD technology.
A feasible improvement solution is to install an MHD driver on a traditional submarine and use battery power. Ensure the submarine's temporary "stealth" when in combat status.