They are useless if you are wanting to know specifically how a pickup sounds, because a pickup on it's own has no sound.
But;
The Stamper videos are very useful to reference the EQ-curve of a pickup compared to another model that a person already has.
I've found his videos very accurate to hear which of the two has more treble, more bass, more mids, ect,,,
Output does not come through the comparisons very well, or at least not as well as it would through a tube amp because that first gain stage of a good tube amp is very sensitive to output.
On the other hand, if one is going to be running boost/od pedals before their amp anyways, then the output differences and the way they affect that first 12ax7 are somewhat reduced.