I think it depends on what you are getting and who it's being offered by... I have purchased several refurbished Hatsan airguns, some direct from Hatsan, others through Field Supply. Some have had problems, others have been problem free. In all cases, Hatsan was pretty good about either refunding, replacing or repairing those that had issues. A few years back I purchased a refurbished BullBoss. I had a multitude of issues and after sending it back to Hatsan twice for repair, they sent me a brand new one as a replacement!
I have also purchased 3 refurbished ATN Scopes direct from ATN. At the time, they offered the same 3 year warranty on refurbished scopes as they did new scopes! I had purchased 1 new one through Optic Planet and a new ATN 5 direct from ATN. Those were the only ones I have ever had issues with. The one from Optic Planet was returned and repaired by ATN and has worked great ever since. The ATN 5 went back to ATN twice, still didn't work right and I finally got a refund. I was going to wait till those were offered as refurbs before I bought one, but I see now ATN is no longer offering the 3 year warranty on refurbs, now you have to pay extra for it!
I also purchased a refurbished AirArms S510 XS from Pyramyd Air. That rifle looked brand new and has functioned flawlessly since I received it!
Refurbs can be a crap shoot, but if you get one from a reputable company, you should get a good one, and if not, still have the backing of a good company to make it right in the end. For the cost savings, I feel refurbished items are generally a god deal!