I have what I guess is a naive question. I have a big 25 hp tractor. By big, I mean the Kubota 2501 is a big compact utility tractor for 25 hp. There are 25 hp tractors half the size and half the weight. My wife has a 25 hp riding lawnmower. On my tractor the backhoe has a subframe it locks into for strength and stability. I am using the backhoe Kubota states is correct for this tractor. When digging, I have the stabilizers down, the front end loader on the ground, and the brake set. When I encounter a large rock, the whole tractor can move. Now, it is probably only and inch or two, but this tells me the hydraulics in the backhoe give this little tractor all it can handle. Can it really be safe to operate a backhoe attached to something smaller than my tractor?
I think that if I had less hydraulic power, I would probably just use a wheelbarrow, shovel, and pick axe. Please don't get me wrong. I am not trying to be a smart arse. I really question if such a device offers something of value. I may be off base on this.
Yes, it can be "safe", it can also be DANGEROUS.
The difference between safety and danger is likely more a function of operator judgement than any (perceived) mismatch between machine mass and attachment forces.
It is a common situation, i.e. many/most back hoes have enough force to pull the whole rig toward whatever you hook into if that doesn't move - an example being ledge.
The machine's mass is usually the limiting factor, not the hydraulics.
Even the BIG excavators have their limits and if you watch skilled operators you quickly realize that they do a lot of work very quickly by working WELL WITHIN the capacity of the machine - not taking the biggest bites possible.
There are rocks my hoe can't lift - OK the usual and acceptable solution is to dig a big enough and deep enough hole beside them and roll them in, i.e. just bury them deeper.
I don't do construction, so it isn't as if they HAVE TO come out.
Clearly SOME rocks do have to come out and that is why you will see VERY BIG machines on some construction sites.
Drilling, cutting or blasting are other possibilities <grin>
BTW, digging stumps (to me) is usually a bigger challenge than digging rocks.
I know I'm close when I can spin it around a bit, but root balls can be incredibly heavy and I have had to PULL out more than I have been able to LIFT out.
At 25 HP I doubt your whole rig adds up to much more than 3,000 lbs, maybe not even that - in which case saplings may be a reasonable challenge, not "Tree" stumps.