In my (limited) experience it is easier to spot bad developers in (perceived hard) niche languages so I would assume a smaller % in that already small group tries to bluff their way in. Learning Haskell, Prolog or something like Coq usually starts on an academic level and if one wants to pursue a carrier it needs to turn into a quite time consuming 'hobby' first if you do not roll into something from uni. In my experience people who are bad are not looking for a career in it while they would be in more mainstream languages (... were Stackoverflow can get you quite far anyway).