The problem with any pickup is that it is amplifying sound coming from only one small part of the instrument.

When you mike say, a mandolin, you are listening to a mixture of soundboard and body, maybe even a bit of strings as well. A Fishman or Marcus Berry or whatever is listening only to the bridge. Sure, much of the sound coming from the strings passes through the bridge on the way to the soundboard, and from the soundboard to the body of the instrument (and back). But you must by definition lose something along the way. That means that, no matter how good the technology, the pickup is never going to sound as natural as a miked instrument.

Having said that, I found over the years that miking was not practical in many of the halls (tin sheds) that we played in. A pickup is needed to reduce feedback problems.

For my Washburn Jethro Burns mandolin I used a Marcus Berry bridge pickup played into a "Passac" preamp.

The Passac was unique in being tuneable. You went through a procedure to tune the preamp to the body resonance of the instrument. This had the effect of re-creating the natural body resonance of the instrument, and gave the most natural sound I have ever heard out of a mandolin. It also reduced feedback. Many fine players in Australia (e.g., Andrew Clermont) have used Passacs from the day they came out, and have sworn by them for years.

I use another Passac on my guitar, with a slimline bridge pickup, and many sound engineers have told me that my guitar produces the most natural acoustic sound they have ever heard.

Unfortunately, I believe Passac were bought out, and the preamps are no longer available; I certainly haven't seen them in shops for years. I am hanging onto mine like grim death!

Incidentally, I have run a Browns Acoustic in my banjo ever since they came out, without a preamp, and it is very good.

Regards
Chris