Theoretical upper limits on the radiation efficiency of reconfigurable antennas based on graphene are presented in closed form and discussed. The theory provides useful information for the design of these structures independently of the type and geometry of the antenna. The case of beam steering, which is very relevant for recently proposed graphene terahertz reflectarrays, is analyzed in detail. Finally, the theory is extended to the nonreciprocal case demonstrating that magnetostatically biased graphene allows the creation of nonreciprocal antennas, but that also in this case there are precise theoretical limits on the antenna efficiency.