String theory is incredibly complexand I can’t claim any great understanding of it, so I can only tell you what little I know:
Firstly String theory as good as it is, is only an extremely promising candidate for a theory of everything, it has not yet been experimentally tested (due to the predicted energies of the strings and of course there is the lack of experimental evidence for supersymmetric partners which appear in superstring theories) and is not yet even fully understood.
In superstring theory there exists a mathematical concept known as duality which allows certain transformations, which means that the different forces can be transformed into each other meaning that you have a basic explantion for all the forces. Also, generally explanations of quantum gravity come up with infinites in measurable situations, string theory tends to avoid these.
Einstein was far from a one-trick pony, he made important contributions in sevreal areas outside relativity, for example it was his explnation of the phtotelctric effect that one the Nobel prize and helped to lay dow the foundatiopns of quantum mechanics.