Generally speaking, I am VERY happy with the Synology RAID-1 NAS I installed about 18 months ago.
However I have noticed that every few months the Time Machine software had trouble finding the Diskstation logical server and therefore mounting the Backup volume I have in there, although the NAS itself is in good health and has plenty of space available. The consequence is that the Time Machine stops trusting the integrity of the backup file and starts a new one, which is a lengthy procedure and loses the ability to go back in time to look for a file I may have deleted.
A little search revealed that – like most networks – I rely on DHCP to assign IPs to the various devices and this may occasionally cause this problem.
The suggestion is to assign the NAS a fixed IP (of course within the subnet addressed by the router) and this seems to have fixed the problem, although I will need to wait a few months to be sure it does not happen again.
Of course, the main root cause is power failures: as I do not live in the city, they do occur every few months: my next task will therefore be the installation of an UPS to avoid hard downs in case of a power failure. I do not have good memories of consumer grade UPS systems but I don’t want to spend thousands; this is the unit I am presently considering, and I wonder how good is the software which performs the orderly down procedure on the connected equipment.