With things mounted on the dash, I've been told that having something that blocks the view of the road and/or comes out further than the steering wheel (looking side on) then that's illegal.
Mate of mine got defected 'cause he had a 2 gauge a pillar mod and the cop said that the second gauge came out further than the steering wheel. So i guess it has to do with safety, dont want to hit your head or if you have air bags it could interfere with them.
Bit of a grey area and as Gizmo said, depends on who you get.