I have had issues with mine in the past. In the 2010 Everglades Challenge off Sanibel ours didn’t update for several hours during severe weather. Probably the time we would have needed it the most.
Here is an article about how the use of a SPOT device left much to be desired. Four people died. Lots of lessons to be learned. There is one very good point towards the end. Any electronic beacon should be regarded as imperfect last resort. Better to manager the risks you take well before you ever need one.