I'm not a aquarium heater specialist, but for my 90l tank I have a Fluval Heater totally submerged underwater, and I just purchased a SuperFish Nano Heater 50w for my gf's Fluval Ebi yesterday and we have that totally submerged, and both my heaters have the minimum water level line, I presume that's so they don't burn out or you want to keep the temperature knob out of the water, or for some reason you can't totally submerge the heater... I purchase submersible heaters so that's what I do submerse them.
I do how ever read all the instructions before I use the heaters, other wise I wouldn't of known not to stick the heater on for at least 5 minutes so the thermostat can correct its temp and then it shouldn't confuse the heater, how ever true that is I don't know, don't bother me much... but no where did I read it cant be totally submerged, it just says on my instructions, submerge the heater above or on the minimum water level clearly indicated and nothing warning me about not totally submerging the item.
If your heater say's ""This heater is not fully submersible" and some reviews claim that it leaks when fully submerged." I would not submerge it, I always read reviews etc before I purchase and check things over, and I hope the SuperFish Nano Heater is as good as the reviews I read, and so far all is ok. I also try to buy with warranty's so at least I can claim a new heater if something broke, but not sure what I would do if all my fish died etc because it leaked or what ever they do. I'm pretty sure it should highlight if it cant be submerged as its electrical, even though you should turn them off, I never do. But I wouldn't be to happy about being electrocuted to death all because the manufacturer was to lazy to highlight "the heater should not be totally submerged" not that it would matter because I would be dead lol.
If I was you I wouldn't submerge it by the reviews you read, sounds a bit dodgy to me, I would email the manufacturer and ask, and tell them if something happen's I want some new neon's lol.