IMO, Marriott SNA is probably one of the most cumbersome and probably useless "benefits" from Marriott. Here is my recent experience.

After you redeem the SNAs for a stay, those SNAs are taken away from your account and tied to that particular booking. But the SNAs are in "pending' mode". So if your reservation is 6 months ahead, those SNAs will be always in pending mode until about one week before actual stay. You can't use those SNAs to try with another reservation. There are very few of those SNAs that they are only useful for one or two reservations. When they are "pending", they can't be used for anything else. When those SNAs were denied, there is little chance that you can use the SNAs again for another booking at the same time.

Whereas with Hyatt TSUs, if any hotel has standard suite available for cash rate, they are available to be reserved with TSUs. Those TSUs are confirmed at once, never in any "pending" mode. Certainly some hotels like Andaz Maui and others play games with TSU. But Hyatt's procedure is very clear and transparent. They are far more useful than Marriott SNAs.