Indeed! iMessage should get an honorable mention. Having lived outside of the US for a few years, sometimes I forget it exists, because here even people who both have iPhones use WhatsApp. iMessage deserves an honorable mention, but with some caveats. As I recall and quickly Googled:
There have been some concerns with their security:
Additionally, iMessage doesn't have any means of out-of-band key verification, so you actually have to trust Apple to faithfully exchange keys and there's no way to verify that it's done so.
iMessage also tells you after a message is sent (via the color of a bubble) whether the recipient received it using iMessage. That's not very good assurance if, say, you're messaging a journalist in an authoritarian country. Will it go out over SMS or iMessage? You can find out, but even a little bit of doubt about that can have significant consequences.
I'm glad iMessage does do encryption like it does, but it's no replacement for Signal and WhatsApp uses libsignal for its encryption.
Yes, and no. If you send a message to someone you've most recently conversed with on iMessage, it will be blue. But if iMessage can't deliver the message, it will fall back to using text messages. I believe on the next attempt, the button will be green, but I don't have a way to test that right now.
As recently as last weekend, I had it go through as green instead of blue without asking because the recipient was in a no-data area. Perhaps because I'd previously approved green messages for that person.
