And if you mess up, even by accident, she will have assassins after you all over Tamriel. Silver-Bloods, you escape Markarth and you are essentially in the clear.
No-one "messes up by accident" Don't get me wrong. It's a bad idea to cross the Blackbriars, but, why would you when it makes so much more sense just do do what they ask. It's profitable and they're great contacts and resources all over Tamriel. What can the Silverbloods offer but slave labor and the occassional thug for hire (or and guerilla's if you feel like slaughtering innocent people). Besides, you would have to be an idiot to "mess-up" by "accident". What, did I pick-pocket her in the Market by mistake? Did I "accidently" break in to her manor and steal her belongings? Did I "accidently" offend her customers and guests at the Meadery? There are no accidents in Skyrim. It's usually because you weren't paying attention. I don't think Maven would have you killed for doing something "accidently". Just grovel and apologize and she'll let you off the hook, although you'll be under her thumb for the rest of your days. Remember Vald? He screwed up major league (losing the pen) and all she made him do was work for Mercer Frey for free as a guard for ever more. I think that pen is probably worth more than anything you could possible pull off as a character.
At least with Maven and her ilk, you know what you did wrong and know that you deserve whatever punishment she has in mind. Besides, she doesn't kill everyone that crosses her. Why bother when extortion, blackmail and jailing to intimidate leaves them alive and able to be used again. Maven actually works to get what she wants (refer to her hiring you and the TG to put her competitor out of business and take over. Nobody got hurt and now she's in control AND you got paid for it).
The Silverbloods on the other hand will TAKE what they want, even if that means killing you or framing you and using you as slave labor and extorting an entire city through fear tactics by using a created "Boogeyman" (i.e. the Foreswarn) that would have, in all likelyhood, never have actually entered the city to attack anyone and would have instead just made the entire region so inhospital to travelers that Markarth would have starved. Look what they did at Karthwasten and during the Foreswarn Conspiracy.