The closest one to me hasn't been used on any trees other than ones owned by large cities. I guess what that means is that "Joe homeowner" probably doesn't value the cost of the technology.
Interpretation on the images was an issue when I last looked into one. That was about three years ago. I am a tech weenie, and if the tools have benefit, or can pay for themselves I tend to buy. Hell right now I am getting my a$$ handed to me by the "grass whackers" turned "tree hacker".
If you can "sell" the usefulness of a resistograph go for it! Just be careful about how you convey the info to the client. Tree risk is a thing that can make the best arborist in the world look foolish IF a lawyer gets involved. Funny, all the ones I know of are owned by ASCA members! They luv sittin' in court! (up here anyway)!