If you're using AMD GPU (I'm using RX 480) you might be unable to use steam overlay while playing AC: Brotherhood.

To fix steam overlay you have to turn off AMD External Events Utility service.

Just press Win + R, type

find AMD External Events Utility and turn it off before launching the game.

Remember: you can't use global hotkeys for AMD utilities (ReLive especially) without this service, so turn it on after playing.

You might experience annyoing micro-stuttering if you enable v-sync in-game. Game is playable but it can be really immersion breaking if your character moves like in slow motion sometimes.

I found easy solution for this:

You have to download MSI Afterburner[www.msi.com] and use included Rivatuner Statistics Server to lock FPS amount.

Just turn on Rivatuner Statistics Server and set global FPS lock to 60.

Launch game with this app on and enjoy stuttering-free gameplay.

Both solutions works for me so I hope this guide would help someone else.

Enjoy your flawless AC: Brotherhood gameplay! :-)