There is no risk on these instant upgrades if your flight operates.
Where you can have a problem is during flight delays, or cancellations or on connecting flights if one your incoming flight is delayed or cancelled.
Let’s say you’re flying DFW-LAX-NHL. if your DFW-LAX is delayed and you miss your LAX-NHL, the next available service might not have N class available, which means no instant upgrade even if the F cabin still had seats.
Your options then are to take a seat in your original economy cabin (not considered a downgrade), or to wait for the next flight where N is available, and F seats are available, so you can ‘instant upgrade’.
I’ve had this happen on complex multi-city itineraries during bad weather. I missed flying to HNL on my planned day as while they could get me in the LAX-HNL, it would only be economy (no thanks for a 5.5 hour flight!)
Other times it hasn’t been much of an issue… flying say DFW-LGA, if your inbound was delayed, usually plenty of other options leaving within a reasonable time where N and F were available, so the delay wasn’t too long.