You could have a bad processor. Hard locks are tough to diagnose without one extra of each item. Hard lock is usually bad component. Memory problems cause reboots, program crashes etc along with the possible hard lock.
This will become much more common. Doctors and lawyers don't like paying for expensive IT. They don't see it as an equal to what they do but it's getting very expensive fast.
I've used pfSense quite a bit and not had your experience. Are you sure its your router and not your modem? Its intermittent so its going to be hard to test. USG works fine but personally I think Ubiquiti's best days are behind them.
Honestly if it isn't a boot drive or game drive you won't notice the difference. Running apps like office adobe will not show much difference at all or being used as a data store will show no difference for normal users.
I doubt you had that many out-of-the-box bad parts. The statistics on that are unreal. However you probably had one part cause others to fail and chain events after.