Anything about propane gets some all riled up about safety... It is the same about any dangerous activity. Guns and electrical wiring are others.
Those that get all riled up usually do not understand exactly how it works, and how to do it safely.
They are correct about one thing: THEY should not do it.
That said, I have been doing the propane refills for 25 years. A complete fill is easier if you freeze the bottles for a few hours.
You can actually over fill them this way. You should really let some out if that happens.. Mainly because the pressure relief may kick it and vent some if conditions change, such as traveling to higher altitude with an over filled bottle... This is why the law says that disposable bottles are not to be transported IF refilled.