Can tonsil stones be harmful?

No! Offensive? Yes!: Tonsil stones are offensive due to odor. They may cause discomfort and possibly and infection could result from them. Mostly it is odor and cosmetic.

Take them out: If you don't have a water pick, get a penlight and a mirror and half-unbend a paper clip. You'll get good at it. Injury / infection risk is minor. If it's truly a nuisance, a otolaryngologist can laser-resurface your tonsils.