Taildrop only works with devices that are yours, i.e. you can't send or receive anything from a device that's either tagged or owned by someone else. The only way malware is getting onto your NAS (via Taildrop) is if you're the one sending it there.
My Tailscale RCE[1] in 2022 involved forcing a device onto an attacker-controlled control plane, at which point your device is no longer "yours", and the fact that Taildrop is implemented in your client binary can be exploited. Chaining the bug to RCE would not have been possible against an ideal Tailscale with only the core feature set.
[1]: https://web.archive.org/web/20221222120703/https://emily.id....
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See my comments in this thread.