This idea that MFDB's are somehow studio devices has been repeated so many times that you can be forgiven for thinking it true. In fact nothing could be further from the truth. I've been shooting on location, up hill and down dale with all my MFDB's since I upgraded from my H5 and H10 backs to the H25. That was a tethered back but Phase had a product the (laughingly) called the "Portable Solution" which was a 10"Sony Vaio on a belt with a couple of batteries to power the firewire circuit. Cumbersome, yes. Stuck in the studio? Hell, no!
Now that we have batteries and CF cards on the backs, there is really no distinction between studio and location work.
I currently field P45+, IQ180 and Aptus 12 backs and they all have been wherever I go. I am an architectural and industrial photographer and also indulge in landscape photography. In the last 18 months, my backs and my trusty Alpa STC have been in the Swiss Alps (both winter and summer), India, Iraq, Afghanistan, Malaysia, Germany, on oil rigs in the Indian Ocean and Arabian Gulf as well as in the Arabian desert. Admittedly, since I've had the IQ, the Aptus hasn't seen much action, but I can assure you, I would have no hesitation taking it anywhere. Naturally it doesn't pay to immerse these things (though apparently freshwater mountain streams are OK for Leaf backs
) but I have shot in snowstorms and tropical thunderstorms with no more than a brolly for protection. The thing to remember is that this level of gear is designed for pro use. That means its designed for gung-ho clowns like me who get paid to ALWAYS get the shot. I certainly would not pay the amounts we do for this gear if I had doubts about its ability to deal with the daily bump and grind of pro use. With some commonsense provisos, this gear really is go-anywhere kit.
The Aptus screen does require some shading outdoors and several options have already been mentioned. I use no more than my hands and the brim of a black baseball cap and have yet to miss a shot because I couldn't read the screen. If you are trying to judge exposure from this screen image (an Eizo it aint!) then you are engaging in a exercise in futility, but all the info you need to compose and expose correctly is there and readily usable.
The Aptus 12 seems to run longer than the IQ on batteries. Power consumption depends on individual use, but relative to the cost of the backs, batteries are cheap. I carry 2-4 and have never run dead on a shoot.
Bottom line is, it's great kit.