The portable ART preamps that I've tested have had high distortion and a haphazard gain structure. Their "overload" indicators are unreliable since they don't sense overload of any circuit stage before the output--I once measured 20+% THD on an ART preamp before the "overload" light came on. A live recording could be ruined and you wouldn't even know it until you got the recording home to listen to.
On the other hand, the phantom powering circuits in the units I've tested have been OK for microphones such as Schoeps and Neumann (i.e. less than about 5 mA), which is better than a lot of other portable preamps in that one respect.
--best regards