I have one on my deck which is not covered in the winter but does have a retractable awning during the summer. I think that anytime you have something burning there is some degree of risk but the design makes it seem like a pretty low risk. There is basically no way that any spark could get out of the burn pot and onto the deck. Also, pellets by their design aren't going to spark and pop in the first place. In theory you could get a fire that burns back into the hopper but based on what I've seen it wouldn't happen as there wouldn't be enough oxygen -- an excess of pellets will smother the fire in the fire pot itself even with the fan trying to keep it running.
The only real risk is a grease fire which the Rectec design works pretty well to avoid. The drip pan does a nice job funneling away. If you do a bunch of low temp cooks to build up a lot of grease (especially if you stuff it and are outside the drip pan) then do a high temp cook without cleaning things you could cause one. Also possible if you are using it for grilling without the drip pan and let a bunch of grease build up down in the body.
In terms of the roof, grease fire aside, there won't be a problem with the heat doing any damage to it. Over time you might end up with some staining from the smoke but even that's probably not all that likely as, outside of startup, there really isn't that much smoke.
Bottom line, I'm comfortable with using it on my deck including for overnight cooks.