It's surprisingly useful. My primary use for it is digging test pits for soil evaluations. I haven't found clay it couldn't dig yet. I dug a few inches into soft bedrock from sandstone a week ago.

I could see heavy clay subsoil in very dry conditions giving it some trouble.

Mine is a 3000 pound machine. I don't think I would want to get one any lighter than that. I've seen some videos of guys adding weights to the back end to improve capability.

Obviously it won't do everything a larger machine can. I wouldn't use it to install a septic system or dig a house foundation, but it does pretty good on small jobs.