iMessage is a huge mess - even if you're still using your iPhone. Back in December, when I was still on iPhone, I went in a month-long trip abroad. Since data roaming still costs a fortune, I had data switched off most of the time. I'd just connect on wifi whenever possible and occasionally switched on the data connection when I really needed to get online while on the go.
Yet, despite the fact that I was roaming and that I had my data connection switched off, most of the text messages that my iOS-using contacts sent me were sent via iMessage instead of plain SMS. Which meant that I'd only get their message when I went back online, hours and sometimes days later.