Hi Norman,
Perhaps my experience is due to not being completely knowledgeable in such matters and thus having taken the risk to overstrain my machine unknowingly, but the scissor knurling tool does not put radial strain on the machine as it floats radially, it only increases the load on the transmission and motor in contrary to the "press sideways to knurl-tools". I have at least not found that it affects the headstock bearings. One must be careful though to take the time needed and not rush the work.
The comparison with the milling cutter is interesting, have not thought about that, but what about a fly cutter which acts on a larger diameter than the exemplified 25 mm knurl, at least my Unimat flycutter is able to span 50 mm diameter. Wouldn´t that strain the little machine more as the single cutting edge whirls around a diameter and thus generates a rotating radial load as well?
I think that it works because it is not possible to create so high loads that the bearings get damaged, the round belt (rope) transmission will bail out long before and act as protection. The headstock bearings are 17 mm inner diameter which is quite a large diameter axle that may handle substantial radial loads, I think far larger than possible to generate on the machine, but I may well be very wrong ...
Well this is my 2cents ..
BR
/Peter