I don't think anyone is seriously accusing Nick of the potential to outright steal the money. He's an escrow, he would be interpreting and executing a contractual agreement between two parties, one of which is his friend and pseudo-business-associate, the other is a British guy in a different country entirely.
There is a valid concern that Nick would favor Ralph if there was ever a technicality over whether the escrow should be paid out or not. We all know how much Ralph loves technicalities. Nick would likely not face any disciplinary issues in a dispute like that, it's a matter of interpretation of the contract. It's not even just a legal issue, this could become a long running mark against his character, exactly like his situation with Ty and #WeebWars. Nick knows this is questionable, which is why he recommended someone else.