FMTV's won awards from the military for their outstanding reliability. They are one of the most reliable military vehicles the US has ever purchased.
That said. Most that are available are 20+ years old, some approaching 30 years old, and the newest ones out there are at least a decade old.
How reliable they are depends on a lot of factors but low mileage trucks have often been neglected because they weren't being used and thus not maintained. Higher mileage trucks have had more "maintenance" but at the same time each time it got touched by PVT Snuffy there's opportunity for it to be done wrong. And there's usually zero consequences for screwing up and only moderate motivation to fix things correctly. They throw parts at problems, don't replace fasteners correctly, overtighten the crap out of everything, don't clean and lube components correctly, and the list goes on and on and on. My truck is a 2008 that the military put a total of about 1500 miles on and they STILL screwed up quite a number things in that short amount of use. There was also zero evidence of any maintenance. All the filters were original from when the truck rolled out of BAE in Texas. And it spent time in Kuwait......
My recommendation for your FIRST truck is to get the newest truck you can afford with a preference for a private party purchase of something that you can inspect and have a conversation about with the seller.