To be fair, when you assess the message reliability of a spot or an InReach, you are testing a device on the move with a constant moving view of the sky. They have to maintain a connection to both GPS and communications satellites in those conditions.
If the device doesn't get a good GPS lock before you start moving, and you are in marginal terrain, it can struggle. The GPS section first has to download the GPS Ephemeris data and possibly the Almanac before it can start deriving locations.
Wikipedia:A position fix using any satellite can not be calculated until the receiver has an accurate and complete copy of that satellite's ephemeris data. If the signal from a satellite is lost while its ephemeris data is being acquired, the receiver must discard that data and start again.
I only know this because I have tested it by starting the spot while on the move in hilly terrain, putting it in my trouser pocket and not stopping until I had turned it off after an hour. Go inside and check on findmespot: 1 beacon instead of 6.
Repeat test: Turn on spot and give it time to lock position, attach to upper portion of backpack harness and repeat the exact same walk. Result: no missed beacons.
When you activate a PLB, it is stationary, and if you follow directions it has a clear view of the sky. It is going to get GPS and Comms lock and it does not have to deal with movement. If you do the exact same thing with a Spot or Delorme you will see similar results. Given the power and aerial on the PLB, it should be better, but maybe not as much as you suggest, it's not chalk and cheese, it's Brie and Camembert