Two or three months ago just before I got my own G3 a few UK members (I'm in UK) reported poorer signal than they'd had with previous phones. I hesitated a bit but then bought and my own tests after buying showed consistently lower signal strength as indicated by the device's dBm/asu measurements compared to those of my Nexus 5, but not enough to matter in my case because I live in an area of good signal. How many users are affected I don't know because we tend to hear only from those with problems, but I think it's generally accepted that the G3's radio is not as good as in some other devices.
I'm afraid I have no knowledge of how it compares to the M8.
Posted via Android Central App