I think it is the artificial sweeteners and chemicals more than anything else that possibly over time could cause complications.
If you have too much of one thing and it's not balanced then there is a risk of something. If you are having one drink of diet soda a day I don't see any worries. Drinking water with the purification chemicals may have a similar affect as everything has something foreign in it these days.
Just live your life and have things in moderation and you will be fine.