They have a higher than usual rate of failure, due most often to loosening of the connection between bus bar and breaker. When this happens, it causes arcing and overheating in the area, which leads to melting of the metal, with breaker permanently welded to bus bar and impossible to remove. Secondary problems are failure of the breakers to trip under an overcurrent event and cases of a tripped breaker continuing to be energized.
The Zinsco company was sold to GTE-Sylvania in 1973, which continued to manufacture the Zinsco panel design under the name of Sylvania-Zinsco. It was also sold under the Kearney brand name. But the design was eventually discontinued, and later Sylvania panels do not have the problematic Zinsco design.