I’ve actually discovered a scenario where labels do work in Collector (though I’m not sure why):
So Collector can do some sort-of labelling, any idea when this will be extended?
That answers it - kind of...
But still: The whole issue about missing labeling on hosted feature services in Collector is a big lack of standard GIS-functionality. And it doesn't seem to have been fixed in the new 10.3 version.
Come on ESRI! What's the problem? If it (unfortunetly sometimes) can work with offline maps, then I suppose it is possibel to make it work all the time.