I am not a lawyer, but from everything I've read, most of these contracts are NOT legally enforceable, UNLESS they are giving you something that is optional such as bonus money, or tuition reimbursement, or maybe a type rating. Something that has material value and/or is portable for employment elsewhere.
Case law strongly has ruled against employers who take the stance that employees should be responsible for training costs, when such training costs are mandatory for the job. That is simply the cost of doing business. As I stated above, only when voluntary (eg. bonus or tuition reimbursement) have the courts been sympathetic to the employers.
Furthermore, the vast majority of these "training contracts" have arbitrary amounts of $$ and for an arbitraty length of time, and are not based on how long one must actually work for them to recoup their initial training investment. The heavy odds are that most courts will not be sympathetic to the employer if the $$ and length of term they put on the contract is nowhere near the actual costs and time to recoup it.
Lastly, in order for them to be able to have an enforceable deliquency to send to a collection agency, they must first go to court to get a judgement against the pilot. Its not like they can automatically and unilaterally degrade ones credit score just because they claim you didn't pay, otherwise almost anyone could do this to those they didn't like.
Again, I am not a lawyer and this is not legal advice at all, but Ive garnered this information from pilots who've flown part 135 and other segments of the industry for many many years