They are over the design weight and have shoddy brakes so they're downrated from their specified top speed. The increased weight makes them much harsher on the tracks and transmits way more vibration to neighboring houses. The doors have overly complex mechanisms prone to jamming. So do the rising stairs. The air conditioners leak condensation. Overall they get far fewer miles between maintenance windows than they should - by less than half - resulting in a lot more cars stuck in the maintenance yard reducing an already over-stretched capacity.

They also can't be coupled more than two cars at a time (where I'm calling a car the joined halves with the flexible part in the middle). Even if Muni had enough cars they couldn't run three-car trains with the Breda units.

SF finally learned its lesson; Siemens is making the new cars. The first ones should start running end of this year. They can be coupled into 3-4 car trains, have good German engineering, working brakes, etc. Muni will also end up with more of them (~120 in the current fleet, vs 240 in the new one) so there will be a lot more capacity.