Been using FreeNAS for over a year. Started on 11.1 train and have been pleased with most functionality. Recently I attempted to run the 11.2beta2, but that ran into complications with temperature monitoring support.
The use of jails was something foreign to me, but BSD has me convinced they are a superior way to run processes on a server. No jail can cause interference with the others and permissions aren't a problem between jails with mounting. Restarting a jail is quick and easy if a process hangs.
One complaint I have is that 9.10 to 11.1 to 11.2 is a strange transition. 9.10 users will be on warden jails, but 11.2 users will be on iocage. 11.1 users like myself are supposed to start using iocage, but the UI doesn't support it and the developers are themselves critical of building jails outside the webui. So at the moment my iocage and warden mixture involved hoping between UI's and CLI's.
Performance with jails has been stellar. VM performance is nice too, but be warned that newer linux kernels still do not play nice with FreeNAS 11.1. It supposedly will be corrected in 11.2 from my understanding.
SMBv1 still hung around in FreeNAS SMB sharing longer than it did in Windows 10, so that caused some temporary complications, but 11.1U6 addresses this by removing SMBv1.
Overall I can't see myself ever using a different operating system for NAS. I'm a ZFS believer now.