Other mistake: When Crickie is typing the letter for Mushu he has ink on his feet yet when he jumps on the table to wipe off his feet there aren't any ink footprints where his foot was.
Other mistake: In the scene where Mulan ties up her hair after she has already cut it, she just wraps the ribbon around her hair and pulls. The way she did it would make her hair fall out.
Other mistake: During the song "A Girl Worth Fighting For", Yao has made several excellent snowmen of women. But the army was marching by, and he wouldn't have had nearly enough time to make them.
Other mistake: When Mushu is getting Mulan ready he ties her hair up, but he never makes a knot so how could it hold her hair up?
Answer: For the reasons stated. He believes him to be a highly competent soldier and trainer. While this opinion is probably influenced by Li Shang being his son, it does prove to be correct, as Li Shang's training is what allows the small group to succeed in battling the Huns.