Unlike Triumph, BSA twins used a plain bush for the crankshaft on the timing side of the motor. Push them long and hard and eventually it was apt to fail. A lot of restorers of these bikes today will convert the timing side to a roller bearing for more reliability.
Don't forget that until 1968 500cc unit Triumph twins also used a bushing on the timing side with just about zero problems. As I recall Gary Nixons 1967 Daytona winning 500 Triumph had a bushing motor.
The BSA's had problems because in 1966 they changed the drive side from a ball bearing to a roller bearing. The change let the crank wander from side to side, which BSA tried to control with shims and a thrust washer, which was a piss poor way of doing things and exacerbated by oiling system BSA used which fed the crank through a drilling in the bushing as opposed to the Triumphs end feed. Basically, it was just a poorly engineered set up, and compounded by a crappy oil pump that tended to warp and leak rather than produce enough pressure. It also allowed the bushing to turn in the case, when that happened oil flow was restricted and soon after the rods broke at the big end and smashed the cases. The ball/needle bearing conversion that SRM made popular is a vast improvement over the stock setup but it also includes reworking the lubrication path to convert it to and feed system similar to what Triumph always used.
Having said that there a still a fair amount of guys in the restoration community that rebuild A50/65 engines using the original style bush, I've watched a friend of mine that does a lot of A65 motors, it generally takes him 3 attempts to get the shimming where has to be. If I ever do another one, I'll bite the bullet and install the SRM kit.