It is very much a word, don't see why or how someone could say that it wouldn't be possible to use that word and I don't think there are any adjectives that can't be negated, well, I am sure there is an edge case or two but nothing really comes to mind. Perhaps неистовый and ненавистный.

https://dic.academic.ru/dic.nsf/usha...BD%D1%8B%D0%B9.