Too much gore, it's time slasher movies came up with better plots other than mutilated humans terrorizing people. Real horror atmosphere can be created when it has realistic touch, like normal people. Also, what type of weird meteorite projects antigravity black tar like that? Why wasn’t the mother affected too? She was in contact with the boys and while she was reading them the witch story, the "magic mutation power holding rock" was right underneath her.
Also, for argument's sake, let's say the rock mutated both the brothers, then why were they immortal? Even if it grossly malformed them and gave them raging appetite, why didn't they eat each other off?