If you get out of this tiny, limited forum and surf the rest of the huge CF site you'll find that Cadillac has a list of chronic problems involving reliability, expense of maintenance, dealership problems, and staggering depreciation.
"Cadillac" is no longer a 30s, 40s, 50s, 60s marque reserved for the well-to-do; rather it's a mass-marketed mass produced GM product that is struggling to maintain the exclusivity of a diamond car with popcorn production. In fact, some of the so-called SUV and crossover models are nothing more than rebranded GMC and Chevrolet models (common in all car lines).
"Worst car by brand" is a pretty subjective title - Given my ownership experience I would have to say, in no particular order, Jaguar, Cadillac, Volvo, AustinHealey and Porsche.