I think it's different from country to country, here in Sweden I think iMessage is reasonably popular, and people here generally go to Facebook Messenger rather than WhatsApp when it comes to cross-platform communication.
iMessage is a very limited and glitchy app when it comes down to it.
Just from the top of my head: no E2EE by default. Gifies are restricted (and censored). Reactions are clumsy (there are two rows of different kinds of emojis to choose from now). Adding photos or sharing ___location is complicated compared to Signal or Whatsapp.
Search is ... well, I hope you don't really need to find anything. Delayed notifications on macOS for no apparent reason, and in 2025 you can still end up with multiple entries for the same contact...
It's all WhatsApp, Telegram and Signal here, nowadays. I.e. they wouldn't know about bugs in iMessage as they never open it.
I'd be curious to hear about other regions of the world. Do people there use iMessage?