I am sorry for the issues you encountered.
But mainly all our customer can use Active Directory integration wihout any problems.
Of course, some may have some issue, but beside identified bugs, it depends on the Network configuration and Active Directory settings.
1) Once QNAP is joined to the domain - we can't login to the shares (QNAP shows up properly in DNS and registers itself as a computer in Active Directory)
2) Once QNAP is joined to the domain - we can't even see the share through browsing by IP address. It will only respond and show shares if access via it's name
3) Slow write performance over cifs/SMB (around 30Mb/s).
1- Do you have any details ? network configuration, specific GPO settings...
By default, domain users do not have access to the shares. You must give some permission.
2- It is the first time i can see this case. Do you have a way to reproduce ? MAybe a Remote connection will help.
Please check your PM, i will manage a remote session to find out the problems.
3- The performance depends a lot on the network environment, and the Models. most of our NAS can reach much faster speed.
Did you contact technical support to fix the issues ?
Maybe if you give us more details we can help here too.