Yes. Even fossil fuels contain far more nuclear energy than chemical energy so the engines we use are ~0% efficient if you actually count that as part of the input, even though we could, in principle, build an engine which does use it. How then do you compare two different nuclear fuels for efficiency?

You don't compare them for efficiency. That's a useless thing to do. You compare for other metrics that actually matter, like cost.

I know. It was just a rhetorical question to further show the unhelpfulness of efficiency.