Against any of the characers that can spam out long range moves his fireball is poor. It relies on MB to be effective as it can knock them back. Rock is too slow and if they are moving, it likely will whiff leaving him open. Pushing back is his strength, getting to be able to do that is his problem. Superman can just sit in the corner and laser vision you far quicker, and he can also mix it up by jumping in the air and doing it. Far better than Sinestro's tools.
Um.... I'm starting to doubt you have even played Sinestro now... that or you may just have extremely slow reaction times.
I've even verified in training just now to be sure, but if you fireball on
reaction to superman's laser at anything more then close ranges you will interrupt it before it hits you. If you are both trying to shoot at the same time you will defiantly hit first and break his laser. At
worst you will trade.
Rock is the hardest move to land properly, but it's best right after they jump, it will hit just before they land (only just clipping them) but in a position where most of the cast can take no action before being hit.
Snare basically you want to use the same way, launch it immediately after they dash (or use a dash move) and it will land a few frames before they can block again.