It's always interesting when someone new to FT comes on to bash a company with their first posts and expects to influence others to not use X airline or Y hotel or Z car rental. Over all my years of traveling, I've had a few problems with all of the "nationally recognized" car rental companies. Stuff happens when you travel. It hasn't stopped me from renting with any of them again. I use Sixt frequently, mostly in Europe and some in the U.S. I've had very good service generally, so despite the OP's entreaty to the contrary, I'll continue to use Sixt.
As AutoSlash said, it would always be better to post details, not just generalities. Great advice!