Yes it does, in fact it is pretty much the canonical definition of something being hard to parse. Given an arbitrary Forth program you can't even tell if the parser will terminate. Granted, the non-fixed grammar starts off simple, but it can be made to be arbitrarily complex.
IMHO "hard to parse" means that writing a parser that works requires a lot of effort. In the case of both Forth and Lisp, that is not the case. Writing a working parser for either language is an elementary exercise, notwithstanding that the syntax can be arbitrarily extended by the user.
