Yeah. Rarely, for my experience, i can only use these bonuses to play slots and i haven't experienced winning even a single round. Though are casino sites that actually gives out bonuses youcan use and cash out on, i haven't yet been to any of those sites.
Casino winnings entirely depend on our luck. So even with these bonuses it quite hard to meet casino requirements to cash out your winnings. Most of the gamblers are not able to meet the requirements and that's why these casinos are running those portions. If all gamblers can cashout bonuses then casinos will soon or later need to close.