Before doing my own, I asked on forums for opinions. Overwhelmingly, I was told not to use them and to hand pack. So of course, I used them. The concern was, as mentioned above, blowing out the rear seal.
I, too, put many, many pumps. I did so until it was obviously clean grease coming out of the front. Key to not blowing the seals, is that I went very slow while my son kept the wheel spinning for me. We spent about 15 minutes on each wheel (that slow).
I was able to see all of the dirty grease come out until it was clean. At that point, I had confidence the job was done.
I've towed about 10k miles since and the rear seals were fine. I'm going to continue to use the system, but carefully.