Sound hole pickups work pretty much like electric guitar pickups, and sound like them too. There are some that are kind of decent, but most do not sound like an acoustic guitar. And string to string volume and tone balance is usually not good.
K&K mini is a passive system, 3 small transducers glued up under the bridge plate, then wired to a simple end-pin jack. You can include a sound hole volume control if you’d like. There is no UST. There are no on-board batteries either. Nothing to permanently alter your guitar, but it will permanently alter the way you think about amplifying your acoustic guitar.
When used with a good preamp (K&K makes a great preamp too) the sound of your guitar come thru, not just the strings.
LR Baggs make a couple of great systems too. And Fishman makes a few decent systems too.
But my overall favorite for true acoustic goodness is K & K.