It's easy to get started, but the issues come mostly from managing that "just a config file" over time.

Have a bunch of new nodes? Replacing a lighthouse? Revoking and replacing certs?

Here's a mistake that I made personally. Did you read the docs fully and realize that the default expiration for a CA is one year? The same is true for certificates. You need some kind of tooling to rotate certs every year, by default, or one day you'll find your entire overlay network disappears.

What about the ACL lists? Well, they're just stored in that same config file. What if you add a new service you didn't count on initially? Or you have a new class of clients?

What if your lighthouse needs to change its IP address? Or you need to retire and replace it outright?

And if you have hosts coming and going a lot, suddenly managing all those configuration files looks like quite a pain indeed...

None of this is unsolvable - assuming you have root on all the nodes you care about. You could even create tooling to automate these things with some kind of configuration management system (which indeed, if you are deploying to more than a handful of systems, you basically must do). But these pain points will eventually add up if you are just trying to connect to friends.