I know that corneal abrasions on the corneas can be bad, but can you predict which ones will go bad?

Infection. Any corneal abrasion that becomes infected prior to healing of the epithelium may cause serious damage to the eye: scarring, perforation.
It depends. Depends on the cause of the corneal abrasion. If it is due to an accidental superficial scratch from a finger nail or piece of paper, these usually heal quickly and leave no long term visual effects. If it is due to an infectious etiology such as a bacterial corneal ulcer or viral herpes, the corneal defect is likely deeper and associated with other corneal findings. These require topical therapy.
Corneal abrasion. The deeper the abrasion the greater the scarring. If it is related to herpes virus the scaring is even worse. Zoster, shingles, can cause scarring in a significant number of people as well.