720p is not too bad on a screen of that size, but the fact is every single phone in its price bracket offers at least a 1080p screen and quite a few brands offer 2K displays for far less. You can actually tell the difference when playing high bitrate videos.
Considering the amount they are asking for, it doesn't make any sense for them to be not giving a higher resolution display. The price difference between a good quality 720p panel and a 1080p one cannot be that high.
Also seen reports that LTE speeds on the XR are slower than the other two models, due to the use of 4*4 MIMO capable modem
Basically they should have released three models, one 'compact', one standard and one Xl, with similar specs, with size being the main differentiator . The XR is not cheap enough to justify the downgrade in specs (no 3d touch as well or the much touted 'taptic engine').