I'm not sad to see the plugins system gone, it was terrible and I always set up my own custom jails on FreeNAS anyway, but It would have been nice to be able to use jails at least from the command line. I guess they don't want to have to worry about supporting both.
I just wonder about the extra overhead of having to run a virtual machine for every container as opposed to just running a FreeBSD jail. bhyve is also not as established as FreeBSD jails and I wonder about stability. Now you're having to worry about the stability of bhyve and docker. For just running services like emby, syncthing, SABnzbd or the like I honestly think having the option to just run a FreeBSD jail is superior. Considering it is FreeBSD under the hood I don't see why the option is gone.
It's nice to be able to run Docker containers don't get me wrong, but I just wonder if it is really the superior technology.