Hydraulic is the strongest and most reliable steering. Worm is OK, but there are many complex moving parts
and many possible shear area's. Pins, bolts, shafts, gears and so on. Then there is the ability to steer the boat in an emergency
steering situation. A mechanical box mounted to the rudder
stock can make that difficult, but this depends on the design of the installation
. I ahve seen vehicle steering boxes mounted the the rudder
quadrant and the steering arm of the vehicle box push/pulls the quadrant just like a ram would. This is a good solution IMO. The worst install I have seen is on a launch where a right angle drive box was mounted to the top of the rudder stock. If anything in the system broke, access to the rudder stock to steer was impossible.
Plus a worn system can allow a lot of play. Then there is the weight factor. I know of several failures of stearing systems that were worm gear
However, there are also alot of these systems out there that work just fine as well. A proper marine
designed/built system would be far superior in all aspects, but I think the reason why this system is so common, is that many home built boats can build an effetive system cheaply by using vehicle parts
to do so.