.... Lot's of prep work ....
I've always wondered about that. It seems like it would be quite a daunting task to get ALL the wood out of a transom when the only access you have is from the top.
It just depends on how you go about it. We removed the inner skin and ripped the wood out from there, do it was a lot easier to get the wood out. I actually used a router to cut up large sections of the wood since I could quickly chew through the wood and set a consistent depth to avoid going through the outside hull/skin.
We have had a number of members on here us it or one of it's competitors (Nidabond, Arjay) and have been very pleased.
I my opinion, it is great for one specific scenario: a boat with a rotted transom that is in otherwise great condition that you don't want to be tearing up. If the boat needs other work like deck and stringers, in my opinion you may as well pull the cap on the thing and put in a properly sealed plywood transom.
Electric chainsaw and shop vac is actually the preferred method.