If I am listed on my uncle's checking account as a joint owner, does that put my personal accounts in jeopardy incase of spend down?

NO unless you have put his money in your account and co mingled funds.

As I understand it.. not if he is NOT on your accounts. Just make sure you don;t put your money in your joint accounts and you have POA and keep records. My Dads money is still his, I can just pay bills, etc and it may help when he passes