I've talked to Larry at FAE about this very issue. He says that they were likewise concerned about swim platform flex when they designed the system, but they've found it to not really be much of an issue. I guess the rubber couplings seem to handle the flex okay.
As far as alternative methods of mouting, if you have a fixed trim tab (where there's only turnbuckles to adjust it or no adjustment whatsoever) they've had successful installs using that as the support brace.
Here are some pictures they sent me: