From what I understand the way this works, the videos and face recognition (anything to do with HKSV) is supposed to be processed on the home hub and stored in your iCloud, not the camera so it should not delete and erase everything. I did the reset over 10 days ago so I couldn’t tell because videos only get stored for 10 days anyway.I’m only afraid on doing these cause it will delete all the stored videos and stored recognition.
You could always save whatever recordings you really need indefinitely to your iPhone camera roll.
As for facial recognition, I have more cameras that weren’t reset so I couldn’t tell you if they aren’t picking up the slack for the cameras that were reset. Once gain, this is processed on the home hub and supposed to be stored in iCloud and technically should not affect it. I would do one camera at a time and see what the behavior is.