The "run-of-the-mill" (average or ordinary / etymologyy) entry on Merriam-Webster Learner's Dictionary contains a reference to "often disapproving". The "garden-variety" (not unusual; ordinary or common / etymology) entry doesn't, but mentions "always used before a noun US". The latter can be found as a synonym of the former; and vice versa, but to a lesser extent.
Are those differences correct and generally acknowledged by the native speaker and are there further differences between "run-of-the-mill" and "garden-variety" I should be aware of (intensity, usage, register, categories of objects it can relate to etc.)?