Word meaning a group of things contains both good and bad.

I think contrasting ( also conflicting) can fit in your second example:

• different, especially with strong dissimilarities , between entities or objects compared.

or mixed:

• Composed of a variety of differing, sometimes conflicting entities: viewed the change in management with mixed emotions; a closet full of mixed outfits.

As for the biscuits I think you have to be more specific saying 'that they were not all good'.

Source: www.thefreedictionary.com

I know "mixed" was given above, but I don't think it's as accurate as

mixed bag (idiomatic) something tending to have both good and bad results or characteristics; something having a mixture of advantages and disadvantages.

The results of the cookie competition were a mixed bag.

Source : http://en.m.wiktionary.org/wiki/mixed_bag