For Barstool, I'm pretty sure you can't just withdraw bonus cash -- you need to play it through, that is, bet it once. But, because Barstool bonus cash does return the stake when bet, and can be split into any number of bets, it's not hard to get out most of its value.

The FanDuel site credit can be split up into any number of bets, unless something has changed in the last couple months.