In general mono black is a very good deck.  The caveat is it's only very good in the hands of a good player.  An inexperienced or bad player will notice the deck under performs markedly so.
Owen Turtenwald with channel fireball has been piloting the decks in the big tournaments I suggest you read his articles.

Some general play advice for the deck though.  It's a 1-1 grind.  You need to appropriately answer all your opponents threats with the 1-1 removal you have at your disposal. If you answer a threat with the wrong card, it could definitely come back to haunt you later. It's a constant check on what's on the board/hand to see if you can maintain the currently state and only using your removal when it's appropriate.  At some point during the game you will be able to turn a corner and take over the match by using underworld connections to bury them in card advantage or desecration demon/pack rat for board advantage.

Also, don't be afraid to go all in on pack rat, it's a strong play in the format and mutavault synergies with it very well.  Against some decks though before you can take over with pack rat you'll get extremely low on life/resources so it can be unnerving at first, but you'll have to get a feel for when it's right to go all in.