The story of flows is a long one.
They have a lot of appeal in the beginner market, and to people who aren't afraid of gimmicks. Just like step-ins of yesteryear, they offer convenience, and "easy of entry" and blah blah blah blah.
Flow has come a long way. There are good riders on them. Their pro riders used to not even rider their product, now a couple of them do. A few still can be caught lurking in traditional strap bindings.
Overall, the nature of the issue is simple: They're bindings you slip your feet into. They have to be loose enough be design to let you foot slip in, which of course doesn't result in a tight fit. To fix this, Flow started offering ratchet strips on their bindings, but at that point you've just got strap bindings really.
In the end... they're just not a solid performance product. People have good luck in the beginner in intermediate skill levels with them, but as you get even moderately advanced, the gimmick starts holding you back. Sure, there are good riders on flows... but here's a video of a kid on a kmart snowboard
So there's people who can make K-mart boads and flow bindings work... doesn't mean they're a quality product.