Most of the people I contact are way less reactive on hangouts (if they ever look at it) than they can be through SMS. I've been a few times in the situation where I realized that I was contacting them through hangouts involuntarily, and that's why they weren't answering. I've never had a Windows phone or IOS, so I don't know how they do, I just hope that it's easier to use
I can't speak for Windows Phone, but iOS displays messages from iMessages -- their private messaging service -- almost identically to normal SMS. (There are some subtle indicators of which one's being used, but it's not obviously different.)
Oddly, the "official" messaging system baked into WinPHone 7 was actually Facebook messages. It didn't switch-over between FB and SMS messages silently, but it did intertwingle the messaging thread between the two platforms and there was a toggle to select which you were using to contact them.
