Loss of Y position on fast jogging.

[cncdrive]

The UC300ETH does not plug directly to the UCBB as a mother/daughter board, because you can use even 2pcs of UCBB with the UC300ETH-5LPT.
If it would plug directly to the UCBB as a motherboard then the option to use 2 of them would be lost and so people who need lots of I/Os would not buy the device.
Or if we were to design the board with all the I/Os which 2pcs UCBB has then the board would be expensive and then people who need only a few I/Os would not buy it, because it would be expensive.

Ribbon cables can pick up EMI only if you run high voltage cables over them in a close proximity. A 250mm or shorter ribbon cable will not pick up any noise otherwise.
If you run the same cable over the board without ribbon cables that could pick up noise similarly. So, you should not put high voltage cables near the board anyways and therfor I don't see any issues with using short ribbon cables instead of a mother/daughter board configuration.

[AvB]

That's excellent info, thanks Balazs. No problems.