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The banking situation is like an MVP of the real thing. You can't easily get a mortgage in another country, some of the banks can't even serve you in English, etc.

As for taxes, in theory, you could have a unified system that exposes a bunch of knobs for each country to turn as they wish. Some might even set some of them to 0. You can do much better than the current mess. I guess you can also do much worse, especially for the people who've already found their niche tax-wise.

Everything is an even bigger mess when you are a company (not even an employer). E.g. recycling mess mentioned here: https://www.pcengines.ch/recycle.htm