If affluenza is real and silver spoons create unproductive people, shouldn't we be wary of spreading a version of it to a much larger chunk of the population?

Yes. He may not be living at the expense of social security but he's consuming resources without producing anything back.

Yes of course, and sending them UBI checks isn’t a good look for the program either.

Or a retiree living off a state pension.

State pensions are the most obvious comparison to UBI with respect to being a fixed livable income paid to people for the rest of their life. The system is sustainable because people have to work before they start receiving their pension, not because pensioners [with intact cognitive facilities] are extremely creative and productive when freed from the need to work to pay the bills.