A lot of barrack room lawyers here. If I ever had any doubts about the legality of a contract I was going to sign I would either consult a legal expert or not sign it. Simple.
As far as the cost of training is concerned, apart from the fact that the above principle applies, there are hidden costs to training which may not be readily apparent to those who give the impression of being new to the aviation industry but I keep coming back to the fact that nobody is forced to sign a contract. If you are moving on, work your contracted notice unless you negotiate an earlier leaving date and pay back any monies that you agreed you would pay back.
Any threat to a person's career comes not so much from this particular company but rather from their own individual actions. Companies who inhabit the lower divisions of the airline game may well be slightly dubious but if you break a contract, you break a contract and any future employer will take this into account. Companies do talk to each other, it's a surprisingly small world and a bad personal reputation will stay with you for a long time. Please believe me, any potential future employer will not be sympathetic to an applicant who has not fulfilled the terms of their employment contract. Not from any particular moral standpoint but because you might do the same to them.