What is easier or more reliable about iMessage? Consider that large portions of the world (among them Germany with its extremely old population) use WhatsApp p

What is easier or more reliable about iMessage? Consider that large portions of the world (among them Germany with its extremely old population) use WhatsApp pretty much exclusively; they don't seem to have a problem using it. Whereas something like iMessage would be a complete nonstarter with its semi-iOS exclusivity.