The only way a man bun can mess up your hairline is if you pull it up too tight. Over time this will cause undue stress on your hair follicles and can cause the hair to fall out easier. If you have a looser man bun then this will take away the stress from the follicle. Some styles and hair types will shave away a couple of mm of hair to create the perfect rounded line, the only trouble is that this will require upkeep and once it’s done you will have to continue to do so.