Extreme cruelty sounds horrible, but it is, in reality, usually a lot more benign than that. For example, “extreme cruelty” could be adultery or emotional abuse, both within the guidelines of extreme cruelty in Ohio.
If an opposing spouse is fighting the incompatibility issue, it may be fairly easy to prove gross neglect of duty and extreme cruelty. The court typically is not interested in making people stay married when one person chooses not to be married, so the court may do what they can to broaden the definitions of both broad neglect of duty and extreme cruelty.