Is there an idiom that conveys the meaning of the French “mi figue mi raisin”?

Solution 1:

A couple of rather-peripherally-related idioms are neither fish nor fowl and "some of this, some of that", neither of which is as close as Martin Beckett's Curate's egg suggestion, but both of which seem more related than phrases like "six of one, half a dozen of the other".

The idiom "mixed bag" meaning #3, "something tending to have both good and bad results or characteristics; something having a mixture of advantages and disadvantages" seems a good suggestion.

Solution 2:

A somewhat similar idiom is double-edged sword

Currently defined in wiktionary:

(idiomatic) A benefit that is also a liability, or that carries some significant but non-obvious cost or risk.

Solution 3:

"Mixed bag" is when the item has aspects some good some bad. If it has one aspect in the mushy middle between good and bad you can call it "so-so".

Solution 4:

Curate's egg is probably the closest thing in English - but it's bit old fashioned now. Nobody (except EL&U readers naturally) would know what you meant.