From what little I know of both, Tailscale provides L2 access into a network that you might not otherwise have access and once you're in you can get anywhere from there, but Boundary hands out individual, already-connected TCP sockets directly to services running on endpoints.
If you're looking for something like a VPN and you're just going to SSH over it, either would probably work for you, but while Boundary can allow users to only connect to port 22 on certain hosts, I think if you wanted to do similar with Tailscale you'd be in iptables/ufw and "tagging / authz-ing traffic with unix uids" territory.
