Upgrading from FreeNAS 11.3U2 to TrueNAS 12.0RC1 went very smoothly, but all of my SMB shares failed to work for Ubuntu 20.04 VMs that relied on mounting them (VMs are hosted on an ESXi platform). The issue was that the user passwords on my TrueNAS weren't migrated from 11.3. Resetting the passwords resolved the issue and allowed my SMB shares to mount normally.
This could be a significant issue if the passwords weren't known, or belonged to other local users. My debug log from 12.0RC1 from before I reset the passwords is attached.