1. Feedback from non verbal communication/behavior is more reliable than, spoken or written communication. 

2. Misunderstanding of non- verbal communication by some individuals have resulted in their living lonely and isolated lives.

3. Deciphering a non-verbal cue is difficult as it's community or culture specific.

A case that comes in mind is direct eye contact. Looking directly into someone's eye during a conversation might be assumed to convey truth telling and respect in one culture, yet in another culture, it might convey total disrespect ( equivalent to glaring or hauntyness) and lack of remorse  in  a different culture! Be careful before condemning someone who looks down as you speak as they may be very truthful and respectful from their cultural up- bringing