I've done a couple of finecast commissions in the past:
http://www.flickr.com/photos/dullspork/6906948137/
The above minis had issues, but they weren't really any worse that I would expect from any resin-cast mini from a garage-kit manufacturer. Most of the issues were not insurmountable. They added some work, but not a great deal more.
My approach would be to:
A: Review the model in question and decide how much work it would be to fix.
B: If it requires a great deal of extra effort then two things could happen - 1. Charge more to fix the issues 2. Attempt to get a replacement with fewer issues.
Obviously, getting a replacement takes time that you may not have.
The thing is, Finecast models are not universally flawed. Some are, some are not. Commissions that include them just need to be handled in their own way. Saying that you won't take them at all just limits your possible income. Most of their flaws can be fixed by a competent modeler. Now and again they cannot. One can always say "no, I'm not going to be able to fix that" once reviewing the issues.