The/a curate’s egg
Definition 1 of 1
Pro Tip 1/3Mixed Quality
Use when something is neither entirely good nor bad, but has mixed aspects. 
This budget hotel was a curate's egg, with clean rooms but unfriendly staff.
Pro Tip 2/3British Origin
Originates from a British cartoon; may be less understood outside the UK. 
I explained my British colleague's 'curate's egg' reference to our international team.
Pro Tip 3/3Not Pure Criticism
It suggests a polite or diplomatic way to criticize by acknowledging some positive aspects. 
Your article is a bit of a curate's egg: great research but with unclear conclusions.