I see no major issue with such purchase.
Purchasing something refurbished always carry the issue that it might have been damaged and then repaired. But, it could also mean that it was returned because the buyer changed the mind about the buying it.
Buying from private sources does not have any recourse at all, in most cases, thus you may be stuck with a defective unit. It is, sometimes, less spendy than through the Canon Refurb channel. One more point here, the unit goes through one more layer of human inspection (fixing/servicing) before certifying for sale. May not be so with brand new stuff.
The only caveat through Canon Refurb is that you don't get to inspect the product after it is shipped to you, you can't open the box, play with the item, decide you don't like it and send it back to Canon.
And, you get some warranty, albeit for a short period.
I bought a 600ex from Canon Refurb and it looked pristine and worked flawlessly. So, I have no reservation going that route again.
Another possibility: buy through LensRental program introduced recently, you can rent and decide on buying, a test drive, if you will. However, with this route, you really don't know how many hands were on the unit before it gets to you. LensRental do a very diligent quality control with every item returned after rental, but just as with any rental, vehicles, houses, etc., there are some less careful people using them.
Best of luck on your choice.
Keep us posted.
-r
PS. Brand new equipment also fails, rarely but happens ... thus, most vendors have some return/replace/service program.