It's because the strut rod is also the torsion bar, so by necessity it doesn't function very well as a strut rod. There's simply not enough cushion at the forward pivot point, which is where the bar turns to go across the car (and becomes the torsion bar). On earlier, straight-bar cars the strut rod has enormous rubber cushions (isolators) at its forward anchor, which allow it to push and pull through its travel, keeping the lower control arm well triangulated. That keeps the steering knuckle straight through the range of wheel motion. On the FMJ suspension, there's simply not enough give at the strut-rod/T-bar pivot isolator, a necessity of the design, since allowing side loading of the torsionally stressed part of the bar could easily cause breakage. Chrysler tried to account for this by adding another rubber isolator at the control arm. Nice try, but it still doesn't allow for the strut-rod portion to act correctly: not only is it still not enough cushion, it also means the strut rod is "floating", meaning there's no solid triangulation point for the LCA. This causes the steering knuckle to go through all sorts of wacky gyrations depending on the wheel travel. Caster & camber both change markedly throughout the wheel travel, which results in handling that is funky at best and downright surprising at its worst. This doesn't happen on the longitudinal-bar cars since the strut rod is solidly bolted to the control arm, very close to the ball joint.
Need more? Look at the length of the strut-rod portion of the FMJ bar. Now go look at any longitudinal-bar car, even an A-body. See how much longer the strut rod is? The longer strut rod travels through a much-wider arc, which means it's not "pulling" on the LCA nearly as much. Even with a similar amount of pivot cushion as an FMJ, the actions on the LCA and therefore the steering knuckle are far less severe.
As if all that weren't enough, now throw all that onto a small mounting platform bolted to the car with four huge, double-sided bushings that allow the front frame rails plenty of room to flex independently. Suspension can't work correctly unless it's mounted to a solid platform, and in the case of the FMJ cars, it's simply not. "But wait!" you exclaim, "Look at the '73-up B-bodies! They have the same cushion setup!" and you'd be right. However, check out the enormous size of the K-member and its mounting points on the isolated B (and '79-'81 R) platform compared with the FMJ. It's a much more stable structure supporting a light-years better suspension design.
Of course, there is also the matter of the "hockey-stick" FMJ bars moving weight (technically load) bias up and forward, which is the absolute wrong way to distribute anything near the front axle if you want the car to handle well.
The FMJ design was considered so bad, a number of Chrysler suspension engineers actually quit their jobs in 1974-'75 rather than be associated with it.
In both suspension designs, cars with polyurethane strut-rod isolators that get driven a lot wipe out the lower control arm bushings much more quickly compared to cars equipped with rubber. Something's gotta take all that stress and it's going to be the LCA bushing every time, which is why other than the manufacturers nobody recommends using polyurethane on the longitudinal-bar cars. I've never heard anything said about the FMJ cars, but very few people are concerned with putting a Band-Aid on a broken leg either. Experience tells me that while one might get the impression that their car handles better with poly bushings on the FMJ T-bars due to several factors (new parts, firmer ride, more road feel), a back-to-back comparison on an alignment rack would show the car will actually handle worse compared to new rubber parts since there's less give and the steering knuckle is now forced into positions the rubber wouldn't allow--in other words, you'll see even-more extreme changes in caster and camber. That means more corrections to make at the steering wheel.
All that being said, if you spend enough time really pushing the limits, you can learn to drive any car correctly so that it will handle in ways that others wouldn't expect. I did things in my Mom's '84 Escort sedan that would make Ken Block poop his pants, and one of my best friends could probably turn Arie Luyendyck's hair white with a '78 short-wheelbase Chevy G-van. You work with what you've got. :icon_biggrin: You learn the faults and foibles and what to expect after a few trips into the weeds or ending up looking at where you've already been... but it's all worth it when one of the local SCCA hotshoes is screaming like a fag choirboy in the passenger seat of your '81 Tercel 5-speed that's held together with drywall screws, bungee cords and a prayer. :eusa_dance: