Too easy to update driver software
Putting all the smarts in the driver that runs on the host CPU that's far far higher performance than the controller running the FTL firmware sounds like a smart idea, except that you're now trusting a driver.
Every time you patch your server, you'll probably be updating the driver, and definitely be updating the OS hooks the driver relies on. It is too tempting for Radian's engineers to keep tweaking for better performance or add features, etc. since you know people will update drivers. With firmware you only fix critical bugs because no one upgrades drive firmware other than to fix serious bugs (except in controlled environments like inside an enterprise array)
I like the idea of keeping the FTL in firmware exactly because it is probably never going to be updated there, unless it is to fix a specific issue. You don't have to worry about patching your server and causing problems with the drive - worst case losing your data. I'm sure Radian will claim they take all sorts of steps to prevent this, but consider in both Linux and Windows there's no memory protection between drivers. If some other driver has a wild pointer and writes in the Radian driver's address space, goodbye data no matter how careful Radian is.
I'll let others take the risk, and think about getting on board after
suckers early adopters have baked it for a few years.