What a camera like the contax does is feel special, or feel better in the hand, with a quality feel. Quality always feels nice and for 1/5 the price of the newer cameras, it's a steal.
At times long cameras like the Contax, Hasselblad and Phase just work differently, i.e. slower than a a flat camera like a 35mm dlsr, a leica S or even a pentax 645 which is almost a flat camera.
Now I love the contax have 5, but rarely use it because of the Leica S2 and if I was buying a digital back for my contax I wouldn't spend past a p30+, never an IQ anything, given how I use cameras.
The only issue with the contax is the viewfinder with a prism seems small, (I've gotten use to it) with the wlf it is huge. No big deal either way and at least you can manually focus with it.
Issue 2 is you must polish all of the contacts, lens, prism, body and back before a week long shoot (though everything should be cleaned on every camera anyway).
To me the Leica doesn't have the sharpness of the p30+ but it's a slightly faster camera to work, and build like a solid piece of billet, but it really does very little the contax won't.
Firstly because a p30 or p30+ will have more sharpness and detail with the contax zeiss lenses than you'll ever need at any size reproduction (if your skilled in post production) and has microlenses which seems to smooth out some of the zeiss lens contrast.
The lcd is limited, but if you work professionally you will be or should be good enough where that won't bother you and if you work for clients your going to tether anyway.
If you really need 35mm dslr type features and shooting experience buy a pentax, but it's a much different camera than a contax and nothing is sharper than the contax zeiss lenses. Their almost too sharp.
The DF+ I haven't held one, but it's new, not tested and I am positive won't do enough to justify the price of $40,000 something for a camera, cmos back and one lens. No still camera today is worth $40,000.
In fact and it's all been said before, but I hear all of this stuff about precision alignment, space aged materials, (i.e. sales talk) and I've never had a focusing issue with a contax and I've owner 4 different backs for mine.
You just have to be careful about keeping the focusing screen flat and never use the phase mask that goes under the focusing screen because it slightly moves the screen up and throws off the alignment, but that's not on contax it's on phase.
In regards to central shutter lenses, you can freeze action a lot easier with short duration flash than with lens shutters.
As you move up line, if central shutters are a must you can buy a demo leica S2 for less than 8 grand and put on any H or contax lens (without cs) with full focus functionality with the leica adapters.
Also once you buy into any phase camera system it takes a strong will to withstand the constant marketing of move up, move up, spend more, trade in, you'll here "be a better photographer if you spend more if not your an okay photographer" . . . but that's all talk, not images.
Now you'll hear a lot about a p30 being a "cropped" camera and that's just more negativity, but it's really not much different in size than most medium format sensors. A p45 for examples has around a 1.14 crop a p30+ a 1.24 crop. Your just splitting hairs.
It's funny you never hear any negativity about the cmos almost 645 cameras that are a 1.3 crop so I wouldn't worry about it.
You'll also hear about "end of life" and "what if it breaks?" Well anyone that shoots for a living needs a backup so try paying for two phase xf cameras and backs vs. a contax and a p30+. Scenario one (new phase one w/ backup is the price of a BMW 7 series, scenario 2 (contax) is a used 3 series and both will work virtually the same.
It's up to you, your money, your life, but if the newest and most advance gets you hot then sony just announced an amazing 42 mpx still camera that shoot 4k video, track focuses and has on sensor focusing and stabilization for $3,200. Now that's advanced tech, but much more computer than camera.
p30+ Contax, 80mm lens f.2.8, 125th second, iso 200, 575 watt HMI
Leica S2, 640 iso, 1/30th second, F4, Contax 55mm lens, tungsten and window light mixed.
I could show more but this should give an idea of the camera's use.
IMO
BC