Yeah, they're not direct replacements. I think both models have have their pros and cons. In fact I tried both around when covid shutdowns started (server being in the office, me at home), and liked zerotier better; it was faster, and a more generous free tier. But now tailscale has won out for a couple of reasons; the main one, it's simply less flaky for us on macOS, especially for devs working overseas. No idea why and maybe there's simple fixes (that don't involve repeated connections/disconnections, hopefully). The other, tailscale has a few extra things that are nicer and easy to use like identity-based ACLs, funnel/serve, magicDNS, ssh management, etc.