Some map apps let you download entire state maps to a phone's storage so complete maps are always displayed even if there is no coverage at all. I don't use Waze but it may do that.
Personally I like having a dedicated GPS. They work with gloves and you can leave them mounted on the bike most of the time.
With a dedicated GPS you should update the maps every now and then. My 5 year old Garmin took me through a lousy part of Memphis last month because the original maps didn't show either I-22 or I-240.