Mods become cheats when they offer accurate information where players without them are left guessing.
A different representation of the same information is fine.
The simplest example would be: Displaying a percentage of health instead of a bar would be fine, but exact health numbers are cheating.
Or: Displaying a warning that a target is using an interruptible ability is fine, but saying what ability that is and exactly when it's going to finish casting is cheating.
Ideally, this sort of information should not be available for modding. Then it can affect neither PvE nor PvP (the latter being the worst affected of the two). If more information being available (like personal damage numbers) is desired, then it should be available without mods, albeit optional.