1. Surface acting is often use to hide your bad mood through smile, enthusiasm and positive talks. It is not necessarily fake but it’s build upon with fatiguing effort(emotionally).
2. Surface acting or any other feeling-undermining activity is emotionally fatiguing and may make your bad mood worse.
3. May be seemed as hypocrite action by others.
4. Extended practice may cause subconscious surface acting and may reduce ability to portray true feeling

Further reading: NYTimes