It has to do with the location of the satellites. The Spot and inreach use the globalstar network if I recall correctly, but they are low earth satellites. PLBs (406mhz) use both polar and geosynchronous sats, so better coverage because the satellites have better view. Plus, to the best of my knowledge, the PLB receivers can do some location sensing even if not getting GPS data, where as the SPOT cannot.
I don't have any data besides anecdotes, and the plural of anecdote isn't data. This may have a lot to do with location, SPOT will have the best coverage in the continental US, since that is where the resources are focused. Where I am in Australia, the SPOT gets much lower reviews, because the angle from the sats is farther. I would also not rely on one in northern parts of canada and alaska as I've used a globalstar phone up there, and it was not at all reliable.
Do you know if your units have a failed send indicator? I've read reports that if it looses GPS while in track mode, it will stop sending updates, but its also a quiet failure, so you need to keep checking to see if its working. Now, I'm not trashing the SPOT, it probably works well. But and that is a big BUT if you need a one button, must work, get me out now button, then a 406 is the way to go. If tracking and messaging are nice things to have, then by all means, the spot can be really handy. This tech has a long way to go yet, and if its working for you where you are, then by all means, go to it. You just have to know the limits of the system.