Your bad grafts may be better than my good ones :)
It is kind of funny. My first grafting came in the form of bark grafting persimmons. I had close to 100% take the first year. In year 2, I tried a few thing to make it even better and had near a total failure. I figured it was my changes. I went back to the original methods and got a so-so year. My experience, for what it is worth, is that factory beyond my grafting technique seem to be much more important. Most of my grafting has been done with a knife. Admittedly, I've pushed the limits with some of the things I've tried, but my success rates have been all over the map.
I doubt this tool well really help my success rates, but I'm interested to try it on some hard to graft species like Jujube. It is very hard wood with a very thin cambium layer compared to pome fruits.
Thanks,
Jack