Nathan Bransford has a great blog post about bragging. And the comments yield an interesting discussion as well.
Bransford’s point is that you should “brag” on yourself all you want in a query letter. Share your relevant achievements and accomplishments. However, then turn off the spotlights. “When it comes to describing the actual work in a query, we really don’t want to be told how great it is,” the agent writes.
So list your relevant credentials and don’t be shy. But don’t describe your book as being life-changing, the best book ever published, or amazing. And don’t say your work is as good as Hamlet
This seems obvious to me, but maybe some writers have no sense of taste.
Besides there isn’t much room in a one-page query for anything but the facts, but writers are creative after all.