The idea is, dissimilar metals don’t like each other. Such as stainless and galvanized will deteriorate each other quicker then brass and copper. so water heaters have special unions that have a small plastic ring that while allowing a seal to be created at the rubber seal, it stops the two metals from touching. So conduit and copper pipes need to be kept separate. Copper pipes shouldn’t touch duct work, and so on. As for the effects it’s more for the connections as far as I know. Also, make sure the copper pipes always end up grounding to the ground. So if there’s a plastic break somewhere along the run, you should jumper the span with a grounding wire.