I was surprised to find sporadic cell service, with my WiFi turned off, on some segments of the Cardinal last year in places I thought had zero chance. Spotty would be the best word to use. But it did exist! Additionally, there were places before 50/51 are in mountainous terrain that we enjoyed service for many consecutive miles.
(Likewise in many places along the Empire Builder's route where only the livestock and wildlife might benefit.)
But where I live in Virginia, near Shenandoah National Park, we struggle for long stretches of driving. And only at a few west-facing viewpoints along the Appalachian Trail do we get service, and then only with Verizon. I guess that could be considered "spotty" too, in the extreme. Go figure...