Domestic civilian and military radar is the same thing. It's (mostly) all managed by the FAA, and the military just gets a copy of the civilian radar feed as part of the Joint Surveillance System. [1]

The limiting factor is geography. Radar coverage in the US is excellent, but the mountains out west make universal coverage hard, and there's some areas that just aren't cost effective to cover.

At 40,000 MSL, yes, everything in the US is covered. Below 10,000 MSL though, there are definite gaps - especially in the mountainous west. It's just physics, the mountains block the signal, and filling in all the gaps is cost prohibitive. Instead, they just tell aircraft to fly higher if they want radar service.

As for outages... ATC radar goes out all the time. You just never hear about it because they either switch to an adjacent, (slower) en-route radar site as part of CENRAP [2], or fall back to procedural control (voice position reports on the radio). Redundancy is baked into the system at many levels, both technical and human.

[1]: https://en.wikipedia.org/wiki/Joint_Surveillance_System

[2]: https://aviation.stackexchange.com/questions/9464/what-is-ce...