So since about 2010ish (Don't quote me) all new PLB's operated on 406Mhz and provides a GPS lat/long. They used to operate on 121.5 Mhz and 243 Mhz (for the military) and there are still old beacons out there using 121.5Mhz. They provide an audible signal only and we frequently get calls in the control tower at work from AusSAR (the organisation responsible for search and rescue in Australia) asking us to see if aircraft can hear a beacon. When you dial up 121.5 you get a very distincntive Whoop Whoop tone that an aircraft with the right gear can home in on.
406 beacons have the ability to be received by satelite and can still be homed on as I believe they still also (again not 100% sure so don't quote me) transmit a signal on 121.5, but also has the GPS which gets the searchers close then the tone guides them to the spot although the Lat/Long is usually on the spot anyway.
So all that being said I run a ResQlink like Alexxx does. I don't use it for mtb but when I'm splitboarding out in the backcountry. I've had it for about 10 years and its had 2 battery updates in that time but I think the new battery is a 10 year life. I've also taken it to Alaska, Kashmir, Japan and the US on snowboard trips.
I don't see an issue with having one for MTB, especially if it gives you or your loved ones piece of mind. They take up stuff all room, weigh bugger all and from my experience in aviation they work.