I'll get things started:
  1) pmc2c.pl is very fragile - when it gets input it doesn't like, it
just ignores it (see RT#39313)
  2) You can't use :slurpy, :optional, or :named arguments. Even if
there's support under the hood, there's no way to write a PMC with
these arguments.
-- 
Matt Diephouse
http://matt.diephouse.com
3) The documentation is poor.  There's a lot of copy and paste coding 
occurring, and that leads to duplication.
4) There's a lot of required duplication.
5) The only code re-use mechanism is inheritance. Roles would be nice.
-- c