Allow me to muddy the waters for you.
" Most mandolins and most 5-string banjos have flat fingerboards, but the necks are so narrow, you don’t even notice."
and
“Since most 5-string players only fret the first four strings most of the time, the neck is narrower, and the strings tend to be lighter, there isn’t nearly as much call for radius-necked 5-strings. However, Deering has gotten enough requests that they have made a radiused neck standard on its Maple Blossom line. On a 1 5/8” fingerboard, most people wouldn’t even notice. But it’s there, in case you wondered. I wouldn’t lose sleep over it one way or the other."
https://creekdontrise.com/acoustic/six_string_banjo/radiused_fingerboard/radiused_fingerboard.htm