arent most of the the tailscale clients open source aside from the gui portion of the non open source os's?
I think the whole Windows client is closed. On macOS though you can use it from the command line just fine (apart from a couple quirks due to a completely different VPN implementation [1]).
[1]: they have three: https://tailscale.com/kb/1065/macos-variants
Yes they are, unless you're using a mainstream OS and/or want to use a GUI, which is probably the most common use case.