There are complaints about Sixt, as well as Europcar. And Hertz. And Avis. And virtually every company out there. Don't discard the complaints entirely, but take them with a grain of salt. True, there are some bottom feeders on the margins, but the majors do a pretty good job overall, or there would be nothing but complaints on these boards.
Out of tens of thousands of rentals every day, a certain number are going to go bad. In most cases, either the renter misunderstood or did not follow the terms of the contract, which are not always the same or enforced in the same manner as in North America, or there was miscommunication somewhere along the line. In some cases, the agent at the local agency was actually dishonest or just incompetent. That is a risk in dealing with hotels, restaurants, shops and any other business.
Most of the time, irate customers blame the corporation rather than the local site, especially if the company does not take the desired action. Companies, meanwhile, have to sort out the legitimate complaints from those by grifters and chronic complainers. And it is easy for anyone with a gripe to pepper the web with "Do Not Deal With Company X" posts that draw others with similar gripes, who seldom post on anything else. But, in the end, they represent only a tiny sliver of the people who rent from these companies without any problems at all.
And, no, I do not work for a car rental company or similar business and my income has never depended on them.