I think that with so many folks choosing track bikes as their platform for urban fixed gear builds you are getting a lot of bikes with short stays, tight geometries, high psi tyres, and rigid frames that are really suited for a smooth track and not the real world.

The nose down / bum up posture is also great for racing but not so good for urban assaults as it puts a lot more weight on the arms and wrists.

Road bikes and touring bikes have longer stays and more relaxed geometry that lends itself to making the ride far more comfortable.

MY fixed mtb is a vintage Kuwahara Cascade which is really a cross country / expedition bike with very relaxed geometry and long rear stays. With 26 by 2 slicks at 60-65 psi it still rolls out pretty quickly and is very comfortable.

My favourite fixed gear (a 1962 Peugeot) is probably the least comfortable of all my road bikes as it has tight geometry, a rigid steel frame, the riding position is fairly aggressive, and I do run slightly higher tyre pressures in the 700:28 rear and the 700:25 front.

I can still ride this bike all day.