I have yet to hear of a single instance of a "training contract" being enforced bonus or not. The legal precedence has already been set, i.e. Great Lakes Case, they don't typically hold up in court even when considering a bonus.
I've read the text of the RAH promissory note and it's constitutionality questionable. The note requires the signor to surrender their right to a trail by jury. That clause in itself is highly questionable, not legal in most states and can likely be successfully challenged by simply telling the court you have changed your mind. No contract supersedes your constitutional rights period. And the fact that such a clause is in the promissory note at all probably invalidates the agreement in whole. But, and there is always a but, how the Indiana civil court system proceeds with this is what will you will have to deal with. Courts, especially civil, don't always follow past precedence or the COTUS with contract law. The note makes you sign an agreement to be subject exclusively to the Indiana court ruling on the issue. That also is legally problematic for RAH as if you are not a resident of Indiana, even if they win and you are no longer employed by a Indiana employer they likely would not be able to collect. For example, they sue you and win, but you work for a Virgina employer, RAH would have to take their case to a Virginia court to garnish your wages and get an "order to pay" and that is where a jury trial would come back into play despite the note you signed. Could they win a 2nd case against you in Virginia or another state? It's possible. But it's not likely they would bother with it considering the complexity of dealing with a second court jurisdiction.
Some would argue that by signing the document you are validating that clause. However, being that if you don't sign the document you don't get the job it can be argued with expert legalese the promissory note was signed under duress.
This isn't simple obviously. But, I have researched this as my nephew is starting at RAH under this contract next month. I have not been able to find a single case filed in the Indiana court system. That seems to indicate that RAH has not been suing pilots over the note. It appears that the agreement and note is a "threat" and not one that they will take action on beyond the withholding of your final paycheck and a letter asking you to pay the balance.
Folks should understand that RAH probably does not want to spend the time or the legal cost in resources and/or funds of pursuing collection of this agreement. Though they certainly can, beyond the final paycheck and nastigram there doesn't seem to be any evidence, at least none that I have been able to find, that anyone has ever actually been sued over the agreement.