...in closed loop the ECU is constantly using o2 feedback to adjust fuel trims, but there are times when the o2 feedback has little value.
1. WOT/Open loop.
2. Throttle transitions
...
I believe those fuel trims are applied to the two cases you reference.
In my opinion, any claims made for these types of products need to be verified and the onus is on the manufacturer. An independent dyno should be used with multiple pre and post runs (with the post runs after a full tank of gas without resetting the ECU / battery disconnect). Engineers can do a lot with an ECU, just look at the 'Volkswagen scandal'. Ask yourself which one wins in ensuring your engine maintains peak performance, a spacer or an ECU.