I beg to differ. This post is highly relevant for people which work with FreeBSD since a not so short while and know about the recommendation to disable the write caches of disk drives to have an acceptable behavior in case of a power-failure. There are many people out there which know about this and disable the disk-cache as one of the first things.
BTW: the handling of the disk-caches os not related to GEOM at all, this is something the ATA/CAM subsystems are responsible for. There was even a short moment in time where sos@ switched the default in the ATA driver to disable the caches by default (IIRC he had to revert it because too much people had the opinion that performance(-reviews) are more important than data-consistency). AFAIK CAM (the SCSI side of it) does not even touch this setting, as it is a property of a drive (a setting in the drive), so you get whatever the drive vendor has set as factory-defaults (most probably the write cache is enabled).