as the name implies, if a PC wants to use an ork weapon, is there any mechanical penalty for using them other than dealing with the unreliable feature?

There's generally some penalties described wherever the specific weapon is described.

I know DH1's Creature's Anathema had ork weapons. Not sure what penalties they had.

Surely it would be an exotic weapon with the penalties for not having the specialism, probably find themselves in Commissar trouble too.

DH1 doesn't seem to have any special rules except that the weapons are Inaccurate and Unreliable in the hands of a human. With an ork they're just Inaccurate.

Creatures Anathema p.102