Time value of money can be solved by adjusting later payouts, and risk of default is meaningless because equity in a failed startup will be worth the same 0 that IOU's from a failed startup would be.
The expected value of a $100k IOU from a startup is likely $10-20k.
The expected value of $100k in equity is (if priced reasonably) roughly $100k even though the value in case of default is $0.
If somebody was really adamant with me about IOUs being treated equivalent to cash, I'd ask if we could simplify and just pay all the founders equally, at the same time to get rid of the discussion.