You'd need to talk to Jason from BMI for an exact description of the changes made to all cars after the prototype , but as far as the battery issue, I know that the battery slot chamfer was changed to alleviate any possibility of the cells shifting. I know that portion of the chassis was re-engineered and any problems solved.
As far as the shafts breaking or bending, I cannot comment as I personally have seen two or three BMI cars run, and make contact with other cars and have not seen anything like this. There was 3 BMI cars running (2 of which either won their mains and/or qualifiers) last night at the track I was at and nothing like that happened.
Email Jason Breiner of BMI at firstname.lastname@example.org
, he can give you more details.
I just ordered 2 cars myself after seeing them run.