I tried using the HF rivet puller a couple of years ago. Went through 2 different tools, neither of which worked properly. Returned them and bought a better tool (Dayton brand if I recall correctly) which has worked well.
However, if I were going to build a complete RV-12, I think I would spend a few hundred $ for one of the high end "Pop" brand rivet guns or equal. I had the chance to use one when I was building my empennage at Synergy Air and the differences were very noticeable. A very smooth pull, lighter and smaller, and with a vacuum holding the rivet in place in the nose piece and pulling the stem into a flexible catch tube. With as many rivets as I think there must be on the 12, I personally think spending the extra money would be worth it.
Good luck with your new project.
RV-10 Finishing Kit & FWF in-progress
2018 VAF Dues Paid