I mean it's a commercial product - if stability issues were a common thing, that would be well known and published.

It's a mesh VPN so it generally has the same stability challenges as any other: Hamachi, Nord, Meshnet, Wireguard (which is the underlying protocol that Tailscale uses).

If the Tailscale client is running on the NAS itself, I'd consider that the first thing to troubleshoot.

NAS's generally don't have much extra processing power, and in my experience apps designed to run on them aren't the most robust. In this case, the VM/dock client has to consume all TS traffic, perform the encrypt/decrypt, put it back on the net (even if it's only with the virtual net) over to Borg, which then has to write to the NAS. The NAS hardware has to do all this, with whatever little bit of ram it came with, also while writing all these changes, managing stripes, etc.

While they advertise these things can run stuff like this, it just seems like an opportunity for performance issues when the processing power design was for a NAS/RAID.

I have an ancient NAS on my network that could probably run the TS client, I just don't bother. TS runs on all desktops and servers (for general access), but connectivity to non-TS devices goes through the server running as a TS router. That can be run on any device running TS - it's just a feature to enable and configure.