In the absence of any other information, we tend to see confident people as more accurate, knowledgeable and credible.
Confidence is the quickest way to assess someone's capability.
The less confident someone is, the more likely we are to seek performance data.
The more easily available is performance data, the more likely we will check it.
When performance data is obtained, it overrides reliance on confidence alone.
A confident person will be more damaged by poor performance data than someone who was appropriately confident.
The prudent, long-term strategy would seem to be to be generally confident, indeed confident enough to say when you have doubts.
(Cheap talk and credibility: The consequences of confidence and accuracy on advisor credibility and persuasiveness. Sah, Moore & MacCoun 2013)