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They didn't work, had to be diesel-powered, overheated in summer with windows that didn't open, had a poor safety record, cost too much etc and so on. Also, he made a big play that they were going to be sold around the world - he went on a world tour and sold zero buses. They're being phased out now, or have been already.
A publicity-driven, costly, embarrassing, dangerous joke. Follow that metaphor as long as it goes.
What was the issue with the new buses?