What, if any, is disadvantage of masturbation; for men and women?

Could be several. Masturbation, though a normal sexual practice and experience, can lead some to guilt and shame and can decrease desire to have sex with another. It can become obsessive and and skew ones normal desires and actions. It can have religious overtones for some. It depends on attitude, mores, religion and ones own comfort with self and if in a relationship, comfort with that too. Doesn't have 2 b any.
Lack of a partner. Not being able to share the experience, providing you are masturbating on your own. No disadvantage if you need sexual release and it doesn't consume your life.