These are engineers, not normal people.

It's fair to expect a professional cook to be able to cook well. Guarding their feelings for their incompetent cooking serves no purpose, since they become useless to their firm, and customers will want to avoid what they cook.

The same expectation should hold for professional engineers. If they are incompetent hacks, tell them they are. Guarding their feelings above describing reality to them only lets their illusion of competence to continue, and is what causes poor software, more incompetent incompetent engineers, and troubled users.