[edit] It boils down to principle of least surprise, managing expectations, etc. - proper documentation is indeed key.
(I'm not impartial about Tailscale.)
Humility helps a lot on the internet- the important thing about iptables is that it runs on millions, possibly billions of machines. Production systems that don't have unit tests but run at scale aren't worse than systems which are newly introduced but have fairly unknown implications.
I'm not making a value judgement about people who need to keep using iptables. I might be making a value judgement about people who demand that everyone else keep using iptables.
But, my point still stands. You can't simply assert your system is better, it has to be proven at a scale similar to iptables before you can say that.
(I'm not impartial about Tailscale, but I don't work there).