I own the 2018 Sierra. I have to say the camera do suck.. Its glitches and the resolution is bad. I don't know why people would argue otherwise unless they are working for GMC, even then why wouldn't you want to hear the truth so you can improve.
I got a 2018 Sierra , after I installed the after market LED backup bulb, the camera would glitches. I brought it to the dealership and they told me it was the LED bulb, so i took it off and put the original incandescent bulbs back and it still glitches sometimes. Listen up GMC!