There are really two issues to deal with... one, The "E3" issues as already mentioned (radio frequency interference.) The night sun uses a carbon arc light and generates a boat-load of RF interference if not adequately shielded and filtered. The second issue is flight dynamics. IFR has very specific stability requirements (discussed in AC 29-1C.) These need to be validated for the specific installation and its potential effects. The basic 412EP with 3-axis SCAS - even with the flight director kit - has no yaw damper. The yaw damping comes in with the 4th axis kit. So when you hang stuff out of the side of basic EP, you could potentially introduce yaw forces inducing tail wag and other undesirable effects that may be minor - but are beyond the stability performance that was the basis of certification. Each installation (STC) would have to be considered individually.