I was Royally stung by a biz jet operator who made me redundant shortly after returning from the type rating course (due to the 2008 financial crash) and they demanded repayment of the training costs. My legal challenge was rejected on the basis that I signed the contract which stated that they could terminate my employment for ANY reason and the training costs would be repayable.
So the answer to the question posed in this topic, yes they are enforceable.