Like any contract, it doesn't matter whether it is enforceable or not........all that matters is who is prepared to spend the most in legal fees.
Generally, repudiating a training bond is bad form and there is a good chance it will bite you in the backside later down the line, no matter how right you are. That said, I see no problem contesting an unreasonable bond as long as you engage through the proper channels and don't just bail.
I might also point out I did get my way with a non-reducing training bond once. There was a bit of legal wrangling for 3 years and we eventually settled out of court for a pro-rata amount which was what I had offered when I left.
I am not a lawyer nor particularly aufait with EU law but you can google liquidated damages. A bond is form of pre-agreed damages(damages = adverse consequences of your ending your contract converted to financial terms). There could also be a penalty clause in order to deter your non-performance but a company would have a hard time using such a clause to deter your leaving. The aim of the contract is to determine reparation for the company when you leave before they have extracted their pound of flesh. In this matter common law may well render any non-reducing penalty as void and would probably impose a pro-rata settlement as that is reasonable. Of course you should consult an attorney as these thing differ from country to country.