Masks worn properly work, masks that don't fit or are the wrong fabric really make things worse.
Issue 1 - glasses. Unless top of mask fits really well they steam up, lots adding to face touching and potentially trips or falls... Many people wear glasses and can't wear contact lenses.
Issue 2. They are hot and uncomfortable. Unless you are accustomed to wearing them and not fiddling, you will touch your face more, including the bit where you've been breathing. If you then touch things (say fridge doors in the supermarket) you are spreading more germs than if you'd not been wearing a mask.
Issue 3. Limited useful life - useless once soaked through by sweat/breath moisture. How many would be required per trip out to remain effective, especially in summer/on the tube?
Issue 4. People acting more reckless because they have a mask on - already seen in supermarkets!