Here's the thing that bothers me (a lot) about SPOT.
Success rates are forecasts for a SPOT messenger to successfully deliver a single message during a 20-minute period, based on network availability. Additionally, in everyday conditions it is normal for some messages to be blocked by your environment, such as hills or buildings. That 's why the SPOT Messenger is scheduled to automatically send multiple messages in every mode – giving you excellent overall reliability
So this means that you might be very happy with SPOT performance because every time you land you can pull up a map with your entire flight, but that's because it's been caching messages and broadcasting a group of them to assure they get delivered once it can see a satellite occasionally.
Put another way, it gives a false sense of security when it might go 5 or 10 or 20 minutes between successful broadcasts. So that day you are in a valley, with limited view of the sky, and have engine trouble, the last successful message might have been sent 20 minutes earlier just before you entered the valley. Thus the SAR will be 20 - 60 miles away from where you actually went down depending on the speed of your aircraft.