Durability and reliability and maintenance are the issues with these type engines.
For some military applications that's not an issue. If you want to build large number of cheap lightweight drones for high intensity warfare, durability is secondary. They will be shot down eventually anyway. Operating smaller number for training purposes during peacetime is not that expensive.
I've had to replace numerous things under the hood due to the heat output though (wiring harness, everything rubber) or otherwise poorly designed (the weak alternators...).
If you discount design flaws in the early RX-8s wrt lubrication, turbo timing issues in the FDs, the oil metering pumps in FCs & FDs, the engines are pretty solid. Almost all of the blown engines I've seen start wth people trying to increase boost.
The problem is that you can modify the car little by little and every way you do this puts stress on the engine. You have to modify the car a lot (fuel pump, injectors, engine porting, turbo wastegate modification, new exhaust, bigger alternator, ecu chipping/megasquirt, cooling upgrades) before the car can really handle the new power that it has without tearing itself apart.
(Of course I think the motor was geared and I think 6000 rpm on the tach was really 2000 rpm, but hey)
After about 60k miles it started getting little puffs of blue smoke when it started up (they went away in a moment).
I got a good deal on another car and traded it in, but I still think about it.
Newer models came along, and finally the RX-8. Now the RX-8 was nice but I believe the last generation RX-7 is one of the most beautiful cars ever.
The engine referred to by my parent poster isn't available anywhere that they could comment on.
I presume that having those seals on the non-moving part is a big win.