Found some answers about the Ranked system in Ubi's official forums:
How does the ranking system work?
The Rainbow Six Siege skill rating system is based on the Trueskill algorithm. It's an evolution of the ELO/Glicko rating systems that works better for multiplayer games (ELO and Glicko are designed for 1v1 games like chess).
Your skill rank is initially computed after 5 placement games, from your skill rating value (for math geeks, the mu variable). Following that, it's updated after each Ranked game you play.
Your skill rank is specific to the region you're playing in (Europe, North America, Asia, etc.). Players from different regions almost never play against each other, and since skill isn't an absolute measure but a relative one (it doesn't tell precisely how good you are -- only whether or not you're better than another player), ranks can't be directly compared across regions (RTS and MOBA players call that the "Korean bronze" effect).
This means that if you move halfway across the world, you will have to go through your placement games again and find out how good you are compared to other players in your new region.
What are the ranks I can achieve?
There are currently 20 ranked levels to work through. Copper, Bronze, Silver and Gold ranks I-IV, Platinum ranks I-III, and Diamond.
Why do I get matched up against players with higher or lower skill levels?
Our Ranked matchmaking tries to match you with people of roughly similar skill. Currently, our matchmaking system casts a wide net, so you may end up matched up with players ranked above or below you. We programmed it this way as one of several measures we took to improve matchmaking times after the results we saw in the Closed Beta. As the playerbase continues to grow and more people are competing in Ranked games, we will make the matchmaking criteria in Ranked stricter.
What makes my rank change? What parameters are taken into account?
Exactly two things:
The relative skill levels of the players/teams in the game.
Whether or not your team won the game.
KDA doesn't matter. Your score doesn't matter. Rainbow Six is a team-based game and as such, the only thing that accurately determines whether you're a good R6S player is your ability to make your team win games.
In addition, if there's a large skill difference between the two teams and the outcome of the game is what the system expects, the skill adjustment after the game will be minimal (if a bronze team loses to a gold team, it's unsurprising and the skill levels won't change much -- now if the bronze team wins, that's a whole different story).
There is, however, one exception to all that: if you abandon a ranked game (whether it's by ragequitting, AFKing or getting votekicked out), it's always counted as a loss for you, even if your team wins.
When the game server crashes or you experience other disconnects while playing Ranked, the intention is that you will not ever suffer a penalty or lose rank. We’re aware of bugs that are still causing issues for some players and are working to fix this in a future update.