The first pattern negates an apparent fault and then introduces a praiseworthy feature as an exception.
In the second pattern praise is affirmed positively first, followed by an apparently qualifying expression that provides more praise:
“I am remarkably eloquent—except that my speech is so clear everyone can understand it.”
The rhetorical effect requires the exception to create a meaningful expectation before delivering the further praise.
It is not equivalent to simply adding one compliment after another; the apparently adverse turn is essential.