Andy Robertson was signed as a first team starter for Hull and that 's the big difference. Joining a club for more cash but a permanent slot in the reserves is not always a good career move. Guys that have left Scotland and done well tend to be the guys who have been signed with the intention of being first team starters. John McGinn is another recent example of that, as is Kipre.
I could be wrong, but I doubt Ben Hall is getting better experience by playing for the worst team in England than he would have received playing first team football for us.
It's all a trade off, though, for longer term gain vs short term cash. And when you are a youngster, the draw of doubling your salary or better must be tempting.
I'd never disparage any player who wants to grab the cash - just suggest they listen to advice from experienced players and managers they trust before they decide.