Really? My experience has been that for some reason, on some phones, messages are only downloaded when the app is opened, which is really annoying but seems to be an interaction-with-the-OS issue rather than a networking issue.
However, messages are always sent and received immediately on phones without this issue (still in my experience).
On my phone (a Nexus 5 running CM), messages arrive promptly on most days, but on some days they only arrive when I open TS. On my wife's phone (different brand) they mostly arrive only she opens TS.
But that's a digression. The point of my comment was that this experience can be rather a turnoff if you happen to notice it really soon after your first install. It's easy to believe that a whole group of people is turned off by one member's negative first experience.
However, messages are always sent and received immediately on phones without this issue (still in my experience).