If you default on the payments after spending the loan, how will they collect? There's no collateral to sell to recoup the principal, and the government isn't obligated to back up the loan you took, only to keep paying you UBI going forward.

Therefore, the loan would have to be dismissable in bankruptcy. So a UBI loan basically reduces down to nothing more than an unsecured "personal loan", which is something you can get today, albeit at very high interest rates:

https://www.nerdwallet.com/personal-loans?annualIncomeFilter...

The average life expectancy of the population would determine together with inflation and interest the maximum amount you'd be able to borrow. A rough calculation at $1,000 UBI a month would give you around $500K available to borrow at 18 and you'd pay back around 750k till you die at 82.