All frames can have issues but the A-body frames are no more prone to having problems than others. I would say less as they are pretty robust compared to many other vehicles.
I've seen quite a few frame sag problems in the 2nd gen F-body cars similiar to the problem in this thread but not A-body cars.
Purportedly, A-body cars ( in certain cases) tend to "relax" under the firewall area where the frame makes an upturn into the engine compartment. Can be aggravated by high torque engines and/or high mileage cars.
Agree on the second opinion. Also, a typical characteristic of a sagging frame is the car won't hold an alignment. But that assumes all the suspension parts are in top shape.
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