14x units couldn't multi with 170s as there was a problem with the electrical side of the coupling. I can't remember which circuits were energised but as others have pointed out, it was for emergency use only.
Essentially the commonality derives from the BSI coupler and the 100ish (I think, its been a while) pins all passing unique electrical signals. So long as all the unit utilises the same basic signals for various functions, they can work together (eg: train wire 4 for the emergency brake circuit). This is a function of the fact that, in essence, theres very little control system differences between a 142 and a 170. Each unit uses a 3 step brake, worked by train wires being energised/de-energised in various combinations. Each unit has 7 power notches etc. The signals all can be passed through irrespective of the physical drivers desk they're being input on.
More modern units do this via a computer interface, and for obvious reasons there will be issues when, for instance, Siemens software tries to talk to CAF software.
You do get some interesting anomalies, however, when units that aren't often coupled end up together. For instance, I remember once coupling a 170 (ex-Scotrail) with a 153 (ex-GWR), which resulted in continuous hustle alarms sounding on the 153.