Agreed. It was only ever intended to be a proof-of-concept for Perl 6, and isn't at all ruggedized against the exigencies of the production environment.

However, it will be core in 5.8 and I've been working on toughening it up for that. You may find it is now far more reliable than it used to be.

In addition, the next release will be layered over Filter::Simple, rather than Filter::Util::Call, and that will definitely make it more robust.