Have regularly sailed on a boat fitted with them for maybe 10 years and always been impressed. They seem to be maintenance and problem free. I would specify them on a new boat in preference to the Blakes on my boat.
I was with my mate "Captain Bligh" at Blagdons Yard Plymouth. There was a steel boat next to his and some clown had fitted bronze through-hull fittings. Oh dear, the steel next to the fitting was eaten away to about half its proper thickness possibly less. The solution is to remove the bronze fitting and clean up the steel. Some people like to grind several radial lines around the hole, at least six. These lines can then be filled with weld so with six or eight radials one is in with a chance. After sanding and painting the steel a new Marelon fitting can be used. Not cheap but you get what you pay for. The material is glass-loaded so it should never crack like a cheap plastic fitting. Good luck.
PS the other advantage of Marelon is that you know what it is. Metal skin fittings are very iffy as they might be brass, bronze, naval bronze (brass) or DZR (dezincifation resistant brass)