The phrase 'Mind you' would have different inferences both by people using it and people hearing it. Although, I'm not an expert teacher, I would be cautious when, where and with whom I use this phrase.
In my view, the inferred meaning depends on the context or the background that it is being used and the familiarity and age of people involved. For instance, when someone is much older or much higher in the hierarchy (boss,supervisor, elders), then the younger individual would avoid using this phrase even if he is correct. For instance, I cannot say to some senior members/ authority figures, Mind you, your replies to my thread are biased or harsh, because it changes the tone of the conversation itself and would annoy the other person.
This phrase can be used to drive home a point or to remind friends or colleagues about an important fact or event. Mind you, tomorrow is my birthday, I except some gift from you. Mind you, if you don't pay your tax by midnight, you would incur a fine. This wouldn't be perceived as a threat.