Your questions, and the things you should be asking, are answered in
[---- 2014/12/24: Note, there is another post describing how to create a full-sized FreeNAS VM directly managing a bunch of disks. THIS post is aimed at people wanting to use FreeNAS to provide a little basic file sharing off of virtual disks on shared VM storage. ----] There's been a lot of...
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In short, your ESXi RAID5 is fine to create virtual disks out of, but it only protects against the VM stalling, and (very importantly) it does not provide data redundancy, checksumming, or repair. For this, ZFS needs to manage redundancy. You can do this by creating a mirror pair of virtual disks, or RAIDZ1 of virtual disks, that are residing on top of your RAID5 datastore.
If you do not create ZFS-accessible redundancy, and bad data is discovered in your pool, it cannot and will not be repaired.