A few years ago it was just impossible to have a factual discussion and to claim that the difference between a D800 and a Phaseone P31+ was small... although we now know for a fact that the DSLR was superior by a significant margin.
In what way? Pure number of pixels? Shooting test targets under perfectly controlled strobe? Looking at spec sheets?
In a more wholistic analysis (without diving too deep into specific use cases):
In favor of a P30+: color (bespoke profiles in Capture One), tonality and transitional rendering, skin rendition, medium format lenses, native leaf-shutter sync speeds, availability of waist level viewfinder, longevity of ownership (we still have P30+ shooters; it was released in 2007), simplicity of interface, AA filter free sharpness, differentiation from a million shooters using D-something-something, speed and consistency of tethering and durability of the tethering port, pro service/support/training, option to use on a tech camera or view camera. Option to use on bodies people have beyond-the-spec emotional relationships, with like the Hasselblad 500, Contax 645 or Mamiya RZ.
In favor of a D800: price, size, weight, very-high ISO, IS lenses, sports-grade Autofocus, availability of very very long lenses.
I'd have thought in the years of this forum, we would have come beyond pixel counting or pure-spec comparisons. I bought a Pontiac Solstice GXP for a few years right out of college; on paper (i.e. per the specs) it was really pretty close to some really nice sports car, and I'd be the first to tell you it was a nice fun car to drive and a great value per dollar, but I'd also tell you in a heartbeat it wasn't a Porsche, let alone a Maserati.
Also note that the D800 is 5 years more recent than the P30+ and I think for what the P30+ did well (e.g. not sports or low-light PJ) it was a much better tool than the D800. A more appropriate temporal comparison would be the P65+ (full frame 645 sensor with 60 megapixel sensor that was widely-seen as better for skin tone, and with a faster capture rate and Sensor+ mode) vs the D800 which favors the P1 even more.
*Technically the P30+ didn't do very well on a tech or view camera, but the similar P45+ and P25+ did.