May not sink a compact, but might break one in half. I've got a Kelly 3ph B600 which weighs about 550# on my Ford 1710 (24hp @ around 2400#). 3ph backhoes are stressful on tractors, and the B600 is about as big as I'd want to go. Even with the relatively small B600, I try to avoid using it really hard.
The B600 has a depth of 6' and an additional 2.5' on the arms would put a lot more stress on the tractor frame. The auctioned unit is about 2 steps larger than my B600. I think many compact 3ph's would lift the larger unit, but I don't think I'd want to use one much, except maybe on a beach.
The specs advertised for most hoes may be a little misleading. Max depths are how low the bucket can physically go. However, actually digging at the max depth is impractical. The bucket is extended almost straight down in the trench close to the tractor wheels, and you can't drag the bucket back further. It's also a good idea not to dig close to the tractor or else you might end up in the hole. A better spec than max depth to follow is depth with a 2' trench bottom, which means that the bucket can be dragged back 2' at that depth. It also means that the tractor has to be repositioned for every 2' of trench.
I suspect that manufacturers are a little generous to themselves with their specs. The depth specs might be given with the bucket extended straight down, which is not the best digging angle. Anyway, trenching at more than 5' with my 6' hoe starts getting impractical. I end up using the wrong digging angles and spend most of my time repositioning the tractor.
