I dispute some of that. Line conditioners rely heavily on power stored in the magnetic fields of their inductors. A half wave gap in input power results in an exponential decay in output during that period as the magnetic field collapses. Where did you get the 'perfectly pass half wave gaps' from? That's untrue.

Not all UPSs use relays, some designs use power switching semiconductors with a far faster switching time and they can respond a lot faster than either a mechanical contact or 1/60 of a second half cycle time which is 8 milliseconds. Any input power dropout obviously detectably occurs within that time framework and requires a response time in the range about 1 to 4 ms, perhaps a 1/4 cycle period.

I dispute that. Some line conditioners rely heavily on power stored in the magnetic fields of their inductors. A half wave gap in input power results in an exponential decay in output during that period as the magnetic field collapses. Where did you get the 'perfectly pass half wave gaps' from? That's untrue.