Wo ist der Haken?
Where is the hook? Where is the catch? The Duden dictionary defines Haken here as a hidden difficulty.
Die Sache hat einen Pferdefuß.
The matter has a horse’s foot. It has a bad side that is not obvious at first.
zu schön, um wahr zu sein
Too good to be true.
die Vorschusslorbeeren
Advance laurels. Praise given before anything has been achieved. The word usually carries doubt.
die Schwarzmalerei
Painting in black. A description that is too pessimistic. The word marks the limit: listing risks is expected, painting everything black is not.