I'm using ScriptRunner's Fields Changed Validator to make sure the Time Spent (the validator notes this as "Log Work") field is changed during a workflow transition. I can see that this validator is accurately preventing the transition from working if Time Spent is not entered on the transition screen, however this validator doesn't tell the user what is wrong - the transition screen just sits there until the user guesses at the field that is preventing the transition.
Using the latest ScriptRunner and Jira Software Server LTS at the moment.