The visibleStart() and visibleEnd() are guaranteed to work only for the following viewTypes:
"Day"
"Week"
"WorkWeek"
The primary purpose of these methods is to provide information on the dates that were calculated automatically. In case of the "Resources" view, columns are set manually and each column can display an arbitrary date. So it assumes you know which dates the calendar displays.
Anyway, the latest sandbox build (8.3.2790) now tries to calculate visibleStart() and visibleEnd() for resources view as well by going through all columns. visibleStart() will return the min date, visibleEnd() will return the max date + 1.
https://javascript.daypilot.org/sandbox/
Just note that it's necessary to call update() after changing startDate (which is used as the default column date) before visibleStart() and visibleEnd() will use the updated value.