There was an article describing how they invented Wordiply, in which it said they decided to accept words which occurred in at least three out of six dictionaries. They said this was to rule out things like names of chemicals. But the implication was a word which only occurs in, say, two out of the six, would not be accepted at all. So I don't know what their criteria are for designating something a rare word. I would have thought "unenjoyable" was a far more common word than any of the examples of Band 3 words in the OED. But perhaps it's not actually more commonly used, just simpler.
Anyway, today's letters are "RIVA" so my first guess was "derivativeness", which is also a "rare word" – and in this case, I'd agree. In fact I'm surprised they accepted it, as they often don't accept words which are, erm, derivations of root words. And it's ugly too!