English Translation
He works fast and is very articulate but his insincerity is his biggest defect.
The overall quality of translation is good, but we don’t say 誠がない（まことがない）. Here are my alternative translations:
彼は手八丁口八丁だけど、不誠実なのが玉に瑕だね。（ふせいじつ）
彼は手八丁口八丁だけど、誠実さに欠けるのが最大の欠点だ。（せいじつさにかけるのがさいだいのけってんだ）
手八丁口八丁, also written as 口八丁手八丁, means “very good at handcrafting and talking” and is used exclusively in a negative context (i.e. criticism). 八丁 (or 八挺) means “eight tools”, so the person is so skillful that he can use eight tools for handcrafting and talking simultaneously" is the original meaning.
玉に瑕 is “a scratch on a precious stone”. It means he is a best-in-class person in a certain area, but he also has a critical flaw.