As a concrete example, a few weeks ago, I invited my dad to my tailnet with the intent of using remote desktop into his machine to help him fix something. He accepted the invite, and then I couldn't ping his machine despite it appearing in my TS domain web interface.
Now he hates tailscale, and I lost credibility because prior I told him how awesome it is. In his view, it wasted his time and doesn't "work right", and metadat is a fool.