Don't forget that iocage itself is still evolving... On the current release of FreeNASS 11.1u4, the iocage version is 1.0 Alpha. So, if one were to say there are still some "gotchas" then I would not be surprised.
I tried iocage back on FreeNAS 9.10 and had some problems with VNET. Tried again in v11.0 and had an issue with updating. Tried again in 11.1u4 and so far so good. Each version of iocage has gotten better. We'll have to wait and see if the next update goes smoothly.
I created my jails by hand rather than using the new FreeNAS interface. The new interface does see the jails, so I'm happy about that - but I don't have confidence yet in actually using the new interface. It is not difficult to set up and maintain iocage jails by hand, and iocage has lots of useful commands to help get all the settings just the way you want. And, yes, there are options for both updating the jails and upgrading them to new versions of FreeBSD.
To see the version of iocage you are running, go to the shell and enter
To see what version of FreeBSD the jails are configured with, run
and it will generate a list of jails showing the FreeBSD version and IP address assigned. I don't think all the jails have to use the same version of FreeBSD (this is something you can designate when you create the jail.)
Note: if you search for iocage documentation, be certain you are using documentation for the latest version. There are quite a few posts of old documentation and comments about older versions - so be careful to use the most current.