I have tried these tools before and I gave them away as it was almost always useless. The success rate is the same as I do it by hand. The only problem we have is high winds during the early spring. The grafts from these tools will always break at the union when the scionwood have leaves and the winds blew. It has a very very very short union, less than half inch to tie the rubber bands to prevent breakage. Unlike my normal hand cut whip and tongue which I can do diagonal cuts of 1.5" to 2" long and then tie around with a rubber band, and it is really really very strong.
My way around to the weak union from these grafting tools is that I use a couple of bamboo BBQ sticks as reinforcements tied with rubber bands to protect the graft union from breakage from high winds that come when the scion have grown. It is more trouble having to brace it, so I gave the tool away as it slowed me down instead. It really doesn’t form a strong union in the early season.
Perhaps if you’re grafting inside a greenhouse, or you can brace the grafts thereafter, then this is a good one.