Five years ago doing this there was no problem and it was all fairly simple to work out. Now, with dynamic pricing and the 5th night free, plus changes in normal T&Cs, I can see the following theoretical risks, but you have no way of really knowing till it happens I believe. Am sure with a customer service contact afterwards something fair could be worked out:
1: Some hotels have a T&Cs at check-in that is really explicit if you change your length of stay they will recalculate your pricing and may charge you a fee. Now in reality for points this pricing change doesn't matter, but if it was a cash stay it would. See this thread: Early Departure Charge . I would worry about this more if you are in a resort or Vegas, not some random hotel in the middle of the US
2: With the dynamic pricing your stay may not work out with the same refund as you expect. For example if you stayed for 3 nights rather than 5 (since you lose the free 5th night, you only pay for 4) you would expect to get back 75%. The "free" night is the cheapest, most of the remaining 4 nights will have some vague variance on points and sometimes it can be significant, so maybe you get back 65% of the points. You can see this when booking, per night. Screenshot it.
3: I'm not entirely sure how this works as in theory Marriott issues a 5 night certificate to the hotel from your account before check-in with a unique number. If the hotel shows an early check-out it needs Marriott to cancel the only 5 nights, and issue a new say 3 night stay retrospectively priced... I have no idea how well the IT works, but in the worse case the hotel just keeps the 5 night certificate and you're out more nights than needed and need an intervention