I've always used bronze thru hulls with bronze seacocks.
Have to replace the starboard cockpit drain seacock.
The Marelon valves seem to be smoother and easier to operate. Even when I greased the bronze ones, they were always stiff.
They may seem that way now but may not once they have been in the water for a while, they may not. There are many cases where the valves have had the handles break completely off. I think the newer ones are better, specifically the 93/OEM series. Forespar requires annual greasing an lubrication something that many just won't or don't do.
Is there any reason I should not put Marelon valve on a bronze mushroom thru hull?
It is a thread mismatch. The thru-hull is NPS (straight thread) and the Marelon valve, if purchased in the US, is NPT or a tapered thread. This goes against ABYC standards though people & builders still do it...
You gain little to no corrosion benefit by threading a Marelon valve onto a bronze thru-hull. If there was a serious DC leak you'd still be in a potential for the bronze to suffer. Personally I find the corrosion issue, IF you use REAL UL Marine bronze parts, is over blown and quite rare. This IS the real benefit to Marelon though!!
I have a box of bronze tapered cone seacocks that came out of a wooden boat built in the 1930's and may have been re-fit in about 1956 but they found no evidence of this. Other than some nasty surface "patina", called Verdegris, which has no effect on the performance of the seacock these valves have ZERO corrosion on them and were in service for at the least 54 years, and if not replaced in the 1950's could be 81 years old.....
In load testing I have conducted the Marelon is considerably less strong than bronze, about half, in a valve on a thru-hull installation...
Valve on thru-hull installations, at least in the smaller sizes like 3/4 & 1", do not meet ABYC strength standards for thru-hulls and seacocks. They should be able to withstand 500 pounds of load at the innermost "hard piping" or the innermost portion of the male adapter/hose barb for 30 seconds. A 3/4" Bronze valve on a thru-hull fails at about 400 pounds, and the same in Marelon fails at under 200 pounds...
If you are buying quality valves such as those by Apollo/Conbraco or Groco the handles may be a little stiffer but they should still open and close for many, many years..
I have no qualms about Marelon IF it is the OEM/93 series, they are excellent seacocks, but the other ones, especially a valve threaded directly on a thru-hull, you'd not personally catch me installing on my own boat...