To overcome the issue of backing up to a Synology, do a couple things (assuming more recent version DSM 6.1.4):
1) Log into NAS, go Control Panel -> File Services -> Advanced Settings -> Change maximum SMB protocol from default SMB2 and Large MTU to SMB3
2) You will still encounter permissions issues. It will continue to cause permissions issues if you just try to assign the DOMAIN\Computer account to the share, so alternatively, add the computer account to a domain security group and then add permissions to said group.
3) From powershell, run Get-SMBConnection and verify under "dialect" variable 3.02. You may have to close out any existing SMB connections and re-open.
This has worked for us, though per Mike Resseler above, you will find some performance issues. I am encountering a lot of ESENT errors for too many threads, no response, etc. on 2012R2 backup server when Veeam for Office 365 tries hitting its datastore on SMB share.
Another method I am finding more reasonable from a performance and consistency standpoint is to set a local target as the backup repository. Then for archiving/multi-point backups, backup said local target to another location such as a NAS using Veeam B&R/Agent.