I upgraded my TVS-872XT to h6.0 beta just before Christmas. For the most part things seem to run well, and performance does seem snappier than it was. I have had some issues with the new ACL 2.0 and NFS though.
The first and most annoying issue was when I decided to upgrade to ACL 2.0. All the folders but one upgraded very quickly. The one remaining folder, which was very large to be fair (27TB), was still trying to upgrade a week later and kept causing access to my shares (particularly via NFS) to lock up entirely. I eventually decided to create several smaller folders and move everything out of the one that was stuck upgrading to get around the problem.
The second annoying issue is that something doesn’t seem quite right with NFS permissions. Before the upgrade, I had an ubuntu server connected via NFS, and the UID and GID matched my user on the Qnap, all new files and folders were created with the correct permissions. Since the upgrade everything is created owned by root. I have tried all the squash users settings to force the right permissions, but that doesn’t help. I’ve had to do a lot of owner and permission changes over the last few days.