1) AOD cars are harder on balancers than 5spd cars typically because the AOD tends to load the motor heavily in the 1100-1800rpm range and there really isn't much you can do about it due to the shift pattern of the AOD. The SC motor has a natural harmonic at about 1300-1600rpm that will cause a vibration that can be catastrophic to the motor if allowed to continue.
2) It has been my experience that once a balancer fails on a motor, it is very likely that it will happen again. This may be due to accelerated bearing wear from the harmonic vibration condition or it may be due to the original balancer failure resulting in a bent crankshaft that tends to break subsequent balancers. This is one of reasons that I tried so hard to get an inexpensive balancer option for us so that people could afford to simply replace an old balancer before it fails. The elastomer wears out, plain and simple, and there is nothing you can do about it other than rebuild or replace the balancer. Clearly the preference would be to replace the balancer.
3) Apart from all of these things it is also my opinion that some SC motors carry a poor balance right from Ford. I recently disassembled an SC motor with 200K miles on it that was on it's original balancer and the parts all checked out to within +/- 1gram. I've seen some SC motors off as much as 15-20 grams. What I'm saying by this is that just because your motor is breaking balancers doesn't necessarily mean anything is fundamentally wrong and needs rebuilding, but you could very possibly be faced with having to pony up for a BHJ.
The kind of bend in the crank that would break a balancer is on the order of .004-.006" so you have to realize that you aren't going to likely be able to measure that with a square of any kind. You'd need a dial indicator at least.
Final note, installation procedure is critical with these balancers. Being that they are aluminum, you cannot force them on! You must use anti seize on the snout and you must use the proper installation tool. A block of wood and/hammer will stress the balancer before you even get it on there making premature failure almost a given.