I used Hangouts for a while and had a bunch of contacts on it when it was Android's default SMS app. Many of them were not particularly technical, including one of my parents whom I don't recall telling to use it. If you were using an Android phone, you were probably already logged in to a Google account. iPhone users had to work a little harder for it (install the app and remember the password to the Gmail account they probably already had).
I don't recall the UX on the mobile client having extra complexity over other messaging apps if I didn't go digging in the settings, but it's been a while.