My guess (only a guess) is that the extended usage of the elec motor in the PHEV is detrimental when towing.
Personally, I decided not to install a hitch on my HEV for this very reason. (All my trailers are heavy.) I bought a cheap, older pickup for my all of my hauling. Whether wise or not, I believe my Niro will last longer.
Also note, when I asked my dealer I was told that installing a hitch voids my factory warranty. Don't know if this is actually true, as my dealer said a LOT of things that are completely inaccurate.