Are subjective standards, such as threats, difficult to evaluate?

Of course they are. That is one of the big problems with the fault system of divorce: it lends to perjury or fakery as well as conclusions drawn from dubious circumstantial evidence, not to mention he says-she says testimony. One could claim cruelty, which would then be difficult to disprove the "proving a negative" problem.

However, at one time, cruelty required actual physical damage to a person. Over time, the effect of the cruelty on the victim became more of the standard than the actual harm. It was determined that one of the reasons for cruelty was to control the behavior of the victim, which often did not require physical contact at all. For example, as has happened, a man who kills his wife’s pet is trying to terrorize her and control her with fear.