It's not just the parachute, the airframe is important as well.
In the case of the Cirrus for example there is a safety cage to protect the cabin and special high-G seats to protect the occupants. It's said that the impact is equivalent to jumping off a 10 foot wall. Now doing that is bad enough when you land on your legs and they bend to absorb the impact. Jumping off a 10 foot wall and landing on your @rse is in a different league.
Collapse of the undercariage and airframe structure will absorb some of the energy if its been designed for it (bit like crumple zones in a car). Just sticking a BRS into an airframe not designed for one won't necessarily provide much protection.
Then again longitudinal strength will come into it. A Cirrus is supported by strops that go more or less to the lower engine mount positions and to a point behid the cabin. On a conventional airframe those points almost certainly won't take upward wrenching loads and the thing might well fold up in the middle.