I had a 68 Mustang 390 with the PS drop bracket and I drove the car for many many years and miles.
Never had any issue with the frame rail getting damaged in any way.
No cracking, buckling, etc.
In fact, the bracket was on the car when I bought it, for who knows how long/miles?
Granted this is an example of one.... but you don't ever, or often, hear about all the good ones, only the failures...
Can't calculate the percentage of "good" to "bad" if you don't have the numbers....
Maybe the folks that have trouble with this are over-torquing the bracket to frame bolts?
There is no question the bracket "must" put more torque/force on the frame rail due to the increased leverage.
But mine held up just fine.
How many that failed had weak, rusty rotted rails?