Back in the day, I owned a few X-frame cars. In high-school, I owned pair of '64 Buick LeSabres. Both had the variable-pitch Twin-Turbine Dynaflow trans. The first one had a stock 401 cid nailhead 4bbl. I didn't know that the car had been in a serious wreck, and that the front-end had been welded back together. One day, I had to do a panic-stop from about 80 MPH when a cow decided to stop in the middle of the road. The front suspension folded, and the wheels came right back into the fender-wells! I swear that I could feel the back end lift off the pavement! That was one hell-of-a ride I don't care to repeat!
I picked up a second LeSabre with a bad engine. I knew where there was a wrecked '65 Riv Gran Sport with the dual-quad 425 cid 'Super Wildcat' nailhead. I bought the car for $35. My hot-rodding buddy & I used his dad's Farmhand-equipped tractor to drag the Riv home, pull the engine & drop it into my LeSabre. This LeSabre had a factory posi rear-end. We put shorter gears in it. With the maroon paint that had faded to a strange purplish color, the car was ugly as hell. It was a real sleeper, though. On a good day, she'd run the quarter in 14.5 sec @ 100 MPH - which wasn't bad for a 6-passenger 4-door back in the early '70s. I taught my high-school girlfriend how to drive in that car. I also taught her some other things in that car, but most of 'em required both of us to sit in the back.
Years later, I owned '60 Chevy Parkwood wagon w/235 I6. It had a three-on-the-tree w/factory overdrive. I used it to haul band equipment.
Never got T-boned in an X-frame car, so I have no first-hand experience with their crash-worthiness. I do remember that replacing the split-driveshaft center bearing in the LeSabre could be a real PITA!! And if you liked to race, it needed to be replaced often!! The rubber mounting couldn't stand up to repeated hard launches. When it let loose, the driveshaft would literally flop around in the center section!!
Buick also did their own thing with frames back then. The 56-57 Buicks used a frame that looks quite similar to the Olds frame from the same era:
In '59, Buick dropped the X-frame completely & came out with this:
However, in '61, they went with the traditional X-frame. Seems like they took a step backward as compared to their Equipoise chassis: