i use NetBsd 10.1, i have exactly same problem, there is no documentation how to mount exfat,
fortunately im able to mout my exfat usb here how i do it
in order to mount exfat with fuse-exfat, you need know it label with
disklabel sd1
find fstype MSDOS
if label is e, you can mount it with this command
mount_puffs mount.exfat#/dev/sd1e <mountpoint>