Ultra Boy's principal weakness is that he can only use one power at a time, i.e., if he is invulnerable, he is not super-strong or able to see through objects. This makes him open to sudden attacks if he is unable to shift from one power to another in time. He cannot use his ultra speed to break the "time barrier" since the stress would destroy his body. He is also vulnerable to "X-radiation", which affects him much as kryptonite harms Kryptonians.