The definition of "unctuous" should have Weyoun's picture next to it. Wiktionary has it as "profusely polite, especially unpleasantly so and insincerely earnest." Although he was sincere in his belief in the Founders.
His limitations, I think, are what makes him so fascinating. For example, his absolute inability to understand why Damar would turn against them. It's as though Weyoun is incapable of reflection. But then, the defective Weyoun 6 was reflective. Perhaps that's what was the root of his defect.