Nothing is simple, but in essence if you agree to something and sign up for it, ie. there is an agreement between two parties, then the agreement is binding. If the terms of the agreement are unreasonable, it can be challenged.
If you agree to go on a course but the condition is that you have to work a year in the company so that the latter can effectively recover its investment in you, and it is all signed up, then that is binding, and probably unchallengeable for all practical purposes, provided 1) you had the option whether to take the course or not, 2) you would not be adversely affected if you were not to take the course (ie. you would not be fired or have you pay reduced), and 3) the contract period was not unreasonable bearing in mind the cost and durantion of the course. (If it was a one day course, a one-year bonding would eb challengable.)
Some courses, such as aircraft type rating courses, might engender a contract period (bonding period) of up to 3 years, but this is getting a bit long and a challenge might succeed. More than this and the challenge almost certainly would succeed.
If someone were to provide a complete ATPL/IR flying training course for an ab initio pilot, that party might reasonably believe that a 4 year bond was acceptable. And so on.
Basically, though, if it doesn't seem reasonable, a legal challenge might be successful.