I would say the problem is really the unavailability of closure iterators. It's
really just something that should not be the case, a language feature not
implemented in a backend. The surprise is caused by a temporary lack of
solutions to this problem, at which point it's good to be wary of such
surprises because there are always problems to be solved. Maybe straight up
saying "closure iterators are not supported" implies some kind of confidence
about the current situation that might be misleading.
