[lmilcin]

You mean this? https://en.wikipedia.org/wiki/Azipod

I think the biggest issue is complexity and resulting unreliability. On a ship that has to be in constant use for decades you want things that are simple and reliable.

"The latest design, the Azipod X, incorporates these improvements, with a view to a service interval of five years, and features bearings that can be taken apart and repaired from inside the pod while the ship is harbored normally"

Yeah, that pretty much sums it up. They are working on it:)

Also an important factor on large vessel is fuel efficiency. You don't want anything sticking out unless absolutely necessary, so these would have to be meant for propulsion. But because of complex construction I can expect they are less efficient than just straight through axle and a huge propeller on it.

I expect things like this to be useful on utility vessels of small to medium size where you don't necessarily need so much efficiency but the utility comes from being able to maneuver quickly and in various conditions.

[gebruikersnaam]

Harbor tugs are more commonly equipped with these: https://en.wikipedia.org/wiki/Voith_Schneider_Propeller

They're quite efficient (although maybe only for lower speeds) and provide extreme maneuverability.

[NorwegianDude]

They usually isn't used as the primary means of propulsion on most bigger ships. It's also not uncommon for them to be retractable. If you need a large ship at a very specific position for a week, then you definetly need it, and that is a common use case as it allows for impressively accurate control. Heck, you could avoid moorings altogether and just keep it in place without it if you wanted to.