Supersymmetry is one (although it's not really a competitor to string theory, more an orthogonal theory).
One of the complaints on string theory, as I understand it, is that it ends up being too general: it sort of implies that just about any phenomenon could be plausible, which makes it really hard to falsify. It's not so much that it's made predictions that turned out to be true, but rather that it can be accommodated to fit prior experiments. From a Bayesian perspective, we haven't received any evidence of its truth, only evidence of nontruth of other theories.
I'm not a theoretical physicist, but my understanding is that supersymmetry is in fact a prediction of many versions of string theory. So it's definitely not orthogonal at all. It's a fundamental part of superstring theory.
This is a complaint rooted in sociology. People thought back in the 80s & 90s that string theory might be so constrained that can only describe one universe (presumably ours). They were wrong. String theory's more constrained than quantum field theory, but it's still capable of describing a lot of different universes. Most of the people who study it at the moment study it because it's been a fruitful source of ideas about quantum field theory. These people don't give press interviews very often, though.
One of the complaints on string theory, as I understand it, is that it ends up being too general: it sort of implies that just about any phenomenon could be plausible, which makes it really hard to falsify. It's not so much that it's made predictions that turned out to be true, but rather that it can be accommodated to fit prior experiments. From a Bayesian perspective, we haven't received any evidence of its truth, only evidence of nontruth of other theories.