I would like to ask, is the IOU written in this situation legally valid?

There are two ways, one is to prove the IOU written while being beaten. According to the contract law, a contract signed under duress is a voidable contract.

The second way is that a loan contract between natural persons is a practical contract, and its establishment is predicated on the existence of a real loan. If it can be proven that there is no loan, the contract will not be established.

If you go to court, what are the chances of winning?

The key is that you have to have evidence. If you have evidence, you will win. If you don’t have evidence, you will lose.