Having replaced a small midge drop bar v braked Pompino with a flat bar compact geared Roadrat a few years back here's my twopenneth of experience.
On towpaths a set of Schwalbe 28c marathons are easilly robust and tough enough but weigh a ton and you really feel that rotating mass. They still cut through mud though so can work in a fashion off road. Spesh Houffalize 32c cross tyres weigh less, puncture more (though not much) and roll a bit slower but allow more comfort and work very well off road and snow etc. 25c Conti 4 seasons will still cope with towpaths and are much faster than the other tyres but still have noticeably more cush than a 23c Conti (as fitted to my carbon roadbike).
So what? Well the 32c was the biggest that would fit the Pompino and still work off road. The Cotic has no such issues (of course disc brakes also great in mud on the Cotic). The On-One had toe overlap even with skinny rubber and mud guards made it a real mare. Again not such an issue with the longer Cotic.
The On-One has more roadie handling and felt much better to ride over any distance generally (more sprightly) and so I miss it in lots of ways. Indeed even my Mrs who also had a similar Pompino but swapped for a bottom of the range geared Trek 1.2 still misses hers.
Now part of the Pompino/Kaffenback weight issue (if there is even a weight issue) is probably down to the forks. A set of Carbon cross forks on one of their frames would be blinding as an inspiring fun bike to ride.
The Kaffenback is just disgusting in the brown.
Cotic now do the X cross bike and I'd love to give that a go as it can be had with a carbon fork and should be less leaden feeling than the Roadrat.
Fixie inc Pure Blood tested some time back in Singletrack would be high on my list if money were not an issue.
My mate had a short drop bar Roadrat (no longer produced) and in the end found it too limited for riding with proper roadies, yet also not good enough to ride and keep up with the MTB rides so sold it on.
So remember if you are looking at something like this it probably falls right between the two schools and so will be better for those solo rides where you just want to explore or commuting purposes. Well unless all your mates are after cross bikes too.
Did the CXSportive in the summer with a mate who has had a Planet X Uncle john for a few years and that was quite a fun alternative to the usual Merida 100/Mayhem/SITS/D2D type enduro stuff we usually do. Also did a cross race on the Cotic once - not for me.