A year ago someone on HN said “I can confirm that iMessage is extremely common in Australia. WhatsApp is very uncommon, outside of people with European (and maybe South American?) friends or family to keep in touch with.”
I can also confirmed that iMessage is basically unused in France. (And that was a core argument in the EU of Apple against the DMA for iMessage, so even Apple admits its low usage in the EU)
The issue with iMessage outside the US is the branding, it's branded as an SMS app and SMS being dead (outside of ads and delivery drivers) doesn't help for adoption.
iMessage is popular in the US because everyone has an iPhone because everyone has iMessage and everyone connects it to social status - network effects. The same reason (besides the social status) everyone uses WhatsApp in Europe.
This has more to do with the way the iPhone was launched, and the American desire to own the most expensive product, than any technical merits.