Personally, I don’t think the Nucleon meets the criteria for being a “flop”. It was just a concept. They didn’t even build a full-size prototype, never mind a production model. By my own definition, the car can’t really be called a flop because there weren’t ever any attempts to sell it.
Cars I do consider flops were the Pontiac Aztec, and if we want to stick to Ford, there’s always the infamous Edsel.
The Nucleon was definitely an oddity, though, and it’s an interesting piece of automotive history. Thanks!