Yes and no. It depends on what you say and how you say it. Given the very strong protections given to freedom of speech an employer has to be reasonably carefull about how he proceeds. Telling your employer the truth is not insubordination. Dismissal would be tantamount to dismissing for honest conduct.

As for the slander, libel or defamation part: There are two defences in law.
1) You are telling the truth.
2) Your comment is what is generally termed "fair comment." As in you have only said what any reasonable person would say.

So if you call your boss an Ass...., you have the defence that he is and if he wants to sue over it, he has to prove that he is not an Ass.....