It is actually worse than some have stated.
Dexter has a video on how this works.... But it only shows part of the story.
The video shows a hub being greased until a dounut shaped ring of dirty grease exits.... Then they proclaim it finished.
But wait a minute... That ring of dirty grease was from the outer bearing. The dirty grease from the inner bearing is still in the hub somewhere. Too purge that will require a LOT of pumping and almost a entire tube of grease for each wheel...until a second ring of dirty grease emerges, Everytime you do it.
Leaving that dirty grease in there would mean that at some point, it will reach the outer bearing, unkown to you.
Unless you have xray vision, you have no way of knowing just what is going on in there