If you take execution out of the equation, and just consider how difficult the characters are to actually play effectively, Leifang and Mila are the hardest, while the rest are pretty straightforward. Sarah can rush any character down with pokes. So can Momiji with her crazy mixups, Christie with her evasiveness and speed, Helena with her stance and huge list of launchers, and Pai with her speed and advantage.
Mila takes a bit more strategy, although her OH makes things quite effective in general since it's impossible to react to whether she's gonna attack or OH you from her transition stance, and generally no attack is fast enough to hit her out of it.
With Leifang you need to know your opponent as much as your own character, because her pure offense is pretty much way below average. She has to rely on counter hits or unsafe setups at almost all times. Her versatility is what makes her good, but knowing how to use her tools effectively is harder compared to the other characters.