I"m no finishing expert but -
1. On steel
(as in cars and steel hulls), lets not forget that all important "conversion coat". That's the one that goes on first which adheres to the bare steel and allows the color coat to stay put. Sort of like the primer which goes under most top coats.
2. Seems to me that clear finishes, like varnish and all the rest, needs to strike a balance between hardness and softness. Must be hard enough to take wear but - especially on wood boats - soft enough not to crack when the wood flexes.
My old woody is all paint except for trim and small areas where good old Flagship varnish is used. 8 coats minimum. Max UV blocking and used only on areas which get no foot traffic. Window trim, butterfly hatches, etc. Just finished redoing one of the butterfly hatches and it confirmed for me that a varnished hull is just nuts.
Did just see a short clip on you tube about a UV cured clear gloss top coat. Hadn't seen that before and don't know the suitability for exterior use.
Botton line is - doesn't matter what the finish is, it still needs to be maintained. The question is, how often does it need to be recoated, how easy is it to repair the occasional ding, how easy is it to recoat.
I've used Cetol DEK for the last couple years and it's an indredable product on bare wood. Not a "marine" product so inexpensive and easy to use.