A large portion of the strength comes from its Logia ability to shift into said element and avoid damage (unless Haki infused, etc.) In addition the user can easily replentish blood in themselves and others. Stop blood flow, change the "state" and type of blood as well as create weapons out of blood.
Cold and Heat can easily effect the user and potientially kill them. Like many Logia a weakness also involves concentration and "skill". If the user is weak the power will be weak and the user will be injured more easily. Another weakness is how much blood the user has, blood replenishing takes time and when using it on themselves it takes longer, not to mention blood types. any of the blood the consumer uses to make weapons has to be out of the body (ie: the blood donor has to be bleeding) or their own. "Healing" can only be used whilst the user is consious. As well as the common water weakness that just comes with being a devil fruit user