Think in terms of torque
Indeed I was. Torsion(al loading): the twisting of a body by two equal and opposite torques. We're both talking about what's happening in the Z-plane, along the longitudinal axis of the chassis & subframe. Correct?
The BH80 is applying a force (lets say 2,000#) at the bucket X feet (lets say 5) from the tractor = a torque of 10,000 FT-LBS that the tractor/subframe has to resist. On the cabbed tractor
(well, the tractor with the Woods vs. Kubota hoe...cab is irrelevant) that distance will be more - not sure if it is 6" or 1foot but the torque would be 11,000 - 12,000 FT-LBS for the same 2,000# digging force at the bucket.
Agreed. To keep things simple, we'll ignore the stabilizers (fulcrums) and consider the 4-pt mount to be the (only) point where the two torques counteract (highest stress). The torque that counteracts the hoe is the basically the weight of the tractor (force) X the distance from the FEL bucket at ground to the 4-pt mount. Now the missing piece ... the counteracting torque the tractor can supply is finite. When the hoe exceeds that guess what happens... the hoe picks the tractor up. Now the point where the two torques meet has transferred more or less to the swing pivot. If the swing pivot is further from the tractor, the torque available to counteract the hoe is greater*... therefore the overall leverage it has is greater, not disadvantaged. This is what I challenged. Throw on the weight of the cab, that advantage increases.
(* Simplification is that the tractor weight is distributed evenly, but depends on where the CoG really is. The weight of the tractor is increased by the additional mass of the longer backhoe mount/operator platform. The operator also sits further from the tractor. Both of these move the CoG closer to the hoe, increasing the counteracting torque. True that if BH were simply moved back further and the CoG unchanged, the BH's leverage would actually decrease).
The subframe is not infinitely rigid & will pass some of the forces/torques to the tractor components/castings.
Agree. In that simplified analysis I'm treating all components as rigid bodies, which there'd better be enough margin designed into them to make that a good approximation.
With larger torques applied, the chance for damage is greater. Weather it is cause for concern, I don't know.
If this were the perceived "disadvantage", then yes absolutely I think we're all on the same page. But let's say the chance was 10% (1 in 10) to start, and it's now 20% worse, should I lose sleep over it being 12% instead?
That would involve a much more detailed analysis with information on the metalurgy of the various parts - subframe, castings etc...
True. Guess I give the guys at Woods alot more credit for designing the subframe beefy enough to handle it. If you compare the Kubota and Woods subframes, you'll see what I mean. Also keep in mind the torque (torsion) in the subframe and chassis decreases the further away from the hoe you get (to zero at the FEL bucket).
The added torque will make it easier to bounce the tractor around with the hoe (pulling against an unmovable stump for example).
Not easier at all.. again, the hoe can only impart as much torque to the tractor as the tractor can counteract. Also if you're pulling straight up on the stump, the craning ability of the BH boom limits your ability way before, say, you can pull the FEL off the ground.
Perhaps the BH70-X is the better choice for the Cabbed
B3030...
Respectfully disagree. Sits just as far back as the BH80-X (if that's the concern), and has only about 18% less power. Its main disadvantage is with the reduced reach... also doesn't have the swing speed regulator which is a nice feature. Besides, I thought downsizing wasn't allowed here 
