I recall reading that there is a tax as well.

As noted in the post, one of the main reasons that people renounce their citizenship are taxes. Wealthy individuals who renounce their citizenship pay, in effect, an exit tax.

One very wealthy person who renounced their US citizenship in favor of UK citizenship was Bill Browder (https://en.wikipedia.org/wiki/Bill_Browder).

Browder ran a hedge fund in Russia that made a lot of money in the 1990s. In 2005 Russia seized some of the funds assets. They imprisoned and killed one of the funds accountants, Magnitsky.

Browder, a UK citizen who had renounced his US citizenship, then went to the US congress and got them to pass the Magnitsky Act, which sanctioned some of the people who were responsible for murdering Magnitsky.

While I think that the law was a good one, I find it ironic that Browder sought redress through the country that had walked away from.

At the time there was a huge amount of Russian money that flowed into the UK and London. The UK, Browder's new country, would probably not have passed a law like the Magnitsky Act since it would endangered all that Russian money.