I can confirm it really IS difficult but not completely impossible. I did book a 5-night standard point reward (i.e., 4+1) last May (i.e., nearly 1y ahead), heading there tomorrow (spending now the 4th night at the Hilton Tahiti which I find not bad at all).

So far no upgrade, and I do not expect any given the above as well as the fact the hotel is fully booked on 2 consecutive nights during my 5-night stay.

Anyway happy to have since directly followed by 6 nights (4+1, +1 additional) on points in CBB. Another 5 nights (4+1) to follow in CBB were released by me last month since travel plans redecided.