Well there are 168 other countries who are not members of the EU and they all have undesirable people but so do the 27 members of the EU.

And million of such (very) desirable people visit the Schengen area each year.

True, but whereas someone from outside Schengen can be blocked from returning for x amount of years, I'm not sure what can be done if someone is from elsewhere in the Schengen zone. Can Spain for instance ban someone and make it a crime if they so much as step foot in the country again, even though they could arrive by car/train without any border check?

And so are Americans - which changes things a bit!

ETA is a UK thing? And the time when there was one big happy Commonwealth family where everyone had freedom of movement all over the Commonwealth is long over since successive British nationality acts. None of British Overseas Territories citizens, British Overseas citizens, British National (Overseas), British subjects, and British protected persons have the right of abode in the UK and all have to apply for an ETA to visit the UK.

T'was in the 1960s. I love how you put it - one big happy Commonwealth family. Absolutely! British citizens, British Overseas Territories citizens, and British National (Overseas) citizens don't need an ETA to return to the UK.

I thought all of the five British citizenships which dealt with overseas nationalities needed an ETA, according to this it turns out BOTC doesn't require an ETA but the other four do.

The most up-to-date rules are here: https://www.gov.uk/eta/when-not-need-eta

British and/or Irish citizens, British Overseas Territories citizens, British National (Overseas) citizens, and foreign aliens with permanent residency or temporary immigration status.