The way this has been explained to me is, chances are very good you will be allowed to reenter.
However they will never guarantee it in writing and in advance because of special unforeseen circumstances. What if you commit a crime while you're abroad but you have a letter saying you must be allowed back into Canada? That sort of thing. So they want to keep their options open.
Having said that, being at the mercy of someone at the border who might have a weekly quota to meet, a bad day at work or anything else is too big a risk for me. Unless I absolutely must, I wouldn't leave until the paperwork for the residency permit is complete. It's a tiny chance, but if that happens, the results are truly catastrophic. Life-altering, really.