People seem to like Sealion scenarios in which the Brits win (I do; see Steven Rogers "Sealion Fails"
http://www.oocities.org/drammos/sealion1.html). It's the Nazi victory versions that stir up the controversy, because it's so hard to come up with a
military scenario in which that would be plausible. I personally think a
political scenario in which British appeasers remain in power and sue for peace to eliminate the threat of Sealion is quite possible, or even one in which the appeasers sue for peace after Nazi troops have landed. Both of these are Failure of the Will scenarios and to my knowledge they have never been fully explored.
The main reason for the
hostility to Sealion threads is the ones that set forth exaggerated views of Nazi prowess. I call these "Provocation scenarios" because they are either launched by (or come under hostile takeover attempts from) people who seem to like the idea of a Nazi victory or, more often, who simply delight in being provocative. In more than one case, the postings of such people have had a troll-like quality. Even when there are no trolls present, the persons who champion the idea of a successful Sealion display an irrationalism (as in ascribing to Nazi warplanes an excellence they simply did not have in reality) and a tendency to cherrypick the facts that remind me of conspiracy theorists.
I am not including ASB scenarios in which the Nazis are given special advantages, or early PODs from our timeline which are intended to explore a more difficult World War Two. These threads don't try to distort the reality of 1940 in our own world and I don't recall any of them becoming the target of the kind of Sealion revisionism that is so annoying.