Drainage and how hedge plants can affect

I have a friend who is about to pull out a big hedge of dogwood because she heard they can cause drainage problems. Would dogwoods cause drainage problems in a garden that would probably not have great drainage anyway? would their roots be particularly binding? the hedge is about 5 or 6m away from soak pit. also, would ash trees cause problems? i read somewhere that their roots go out quite a bit further than other trees.