Why did so many instances use .ml domains?
'Cos they were free.

Ahh, makes more sense.

Are there any other domains that could be in a similar situation now?

At the time you get .ml free, .TK, .GA, .CF, .GQ were also available for free (they ended up all being used by spam advertisers, so those domains get marked ‘suspicious’ a lot)
Niue is trying to wrangle control of .nu form Sweden, so I’m a little worried. It’s been ongoing for years though.