One of the sources of this problem is the relentless experimenting that has been ongoing since about 2007-2008. First, there was pmount and hal introduced to take care this automounting, and as soon as this was in a working state, it got replaced by devicekit. Soon after, devicekit was abandoned in favor of a combination of udev, consolekit, policykit. Then then policykit got replaced by polkit. And then systemd was introduced and consolekit got abandoned. And each time the documentation gets worse (except for systemd, since the authors try to justify their choices).

Overall, it has been a sequence of constant rewriting, and I don't see an end to it. People who have been using rolling distributions like Gentoo and administer their own machines are the most affected. If one is not using the whole systemd to major DE (KDE, Gnome, XFCE, etc) stack, it is arduous to set up a mounting system which always works. Usually, vfat external disks tend to be immediately writable on mount. Move over to exfat, mtp, ext[234], or another other native filesystem and it is an exercise in exasperation to get them to mount as read-write as an unprivileged user, or even mount at all as an unprivileged user (see https://bugzilla.kernel.org/show_bug.cgi?id=15875).

The only decent program that I have found that works quite reliably is udevil.