Any bag containing a mixture of something.
a mixed bag of nuts
By extension, a group of entities with few characteristics in common; an assortment.
The people I met at the support group were a mixed bag: some were grouchy pessimists, and others were cheery do-gooders.
Something tending to have both good and bad results or characteristics; something having a mixture of advantages and disadvantages.
Owning a car is a mixed bag. It is convenient, but also costly.