When I had an iPhone I had major problems with iMessage. As I had plenty of texts I just turned iMessage of and used traditional SMS instead.
The biggest issue I had with iMessage was when I had poor mobile signal. It would decide that as I had a flaky GPRS connection that it would use iMessage and then 10 minutes later it would fail. But you couldn't easily force it to SMS - and SMS would work fine because it doesn't need a data signal to use it.
Either that or just use one of the other cross platform messaging solutions (like whatsapp or Facebook messenger or whatever) and toss the iMessage rubbish in the bin.