On the other hand, our use of their data has been fairly unpopular since it sometimes stretches people's conceptions of what is an urban area.
There is an alternative – but it's bad. The UN maintains a list of metro areas in its "World Urbanization Prospects" report. Unfortunately, this hasn't been updated since 2017!
Wikipedia still uses the 2017 data which means their list of biggest cities is frozen in time as well.
Very frustrating all around.
Edit: They did update it! But for some reason they showed an old cached version to me. Once I opened in incognito mode I saw the new version. Weird. They have a very strange tech stack on that site.
The actual city populations, though, are a hit-or-miss. Most non-Western countries are hilariously bad at reporting data.
Same for China, where some people say the real population is closer to 1.2 billion than 1.4 billion.
Heck, even in the US, we don't know the exact numbers. Are there 5 million illegal immigrants, 20 million? Nobody really knows.