I've had over 75 nights each of the past two years, and selected 5 SNA for both 50 and 75 nights. I've been very hit and miss on the upgrades, and it's frustrating.
On one, I got confirmed for the upgrade, then when I got to the hotel they tried to tell me that the upgraded room wasn't available. After about an hour talking with them, talking with Marriott CS, and talking to the manager, they "found" a room.
On another, the upgraded room showed as available 6 days before check-in, 5 days before (when it should have confirmed), and 4 days before. But I never got my upgrade. It disappeared at 3 days out. I cancelled and rebooked at a different hotel and was able to get an upgrade (without having to use my SNA).
Most of my others went through. For some that didn't, I booked a second room.
The SNA awards are a huge perk for me, and it keeps me fairly loyal to Marriott. (The fact that I'm lifetime Platinum helps, too, but I've been staying enough to hit Titanium.) With a family of 5, used to staying in timeshares, a 2BR is almost a necessity. Most of my travel is without the family, but when I travel with them, we want plenty of room.