Neither BU nor Garmin can be used without drivers.
IF your BU or PC pack up, you have no backup. If PC packs up, you have your Garmin as a backup GPS/plotter. +1 for Garmin then. And it can be run on batteries too. +2 for Garmin. And you can keep it in the pocket and use in the rain / plying spray. ETC.
To feed AIS
into OpenCPN you will need a nmea
(serial) to USB cable, unless you buy a USB out AIS receiver.
We use a SH radio
for AIS (small AIS screen
but forwards AIS NMEA
data to PC) and Garmins (72/76) and a netbook with MaxSea
/ Deckman / SeaClear / OpenCPN (testing only) and we also use BU. All the above and more interface easily and without issues.
This much said, on a passage
I use most of the gear
IMHO the biggest bonus is ability to plan the route
on your PC and transfer the WPTs to the plotter (handheld Garmin, in our case). Saves loads of time and probably some errors vis transferring the data by hand.