hard truth cuts both ways

Stannis Baratheon says this to his commander.. I can't remember his name .
I am not a native English speaker. And i am eager to learn this phrase
Thank you. cheers

In this particular episode, "hard truth cuts both ways" mean: it hurts both sides (directly implied, or involved) equally.

Stannis and his Onion Knight, Ser Davos Seaworth (Liam Cunningham) who the reality of their situation injured them both!!!

(in English it can also sometimes mean having disadvantages and advantages).

This article explain this particular part of GOT pretty well:
http://www.televisionaryblog.com/2012/04/hard-truths-ghost-of-harrenhal-on-game.html

Hard truths are that which would not otherwise be spoken imo. They hurt the person receiving the information, and the person giving it.

You guys excited for Season 7?