Yes many do not pay the credits.
Check the rating they get, I would not even consider doing a free one unless it has a 5 start rating and even then it's not guarenteed.
As for free credits, it is very possible and easy. This may not work for people outside the UK, because the offers maybe different. This is what I did:
There is a betting company called William Hill they had 3 dif offers.
1) Deposit £10 at sport betting. Only requirement is you bet. You can bet in play, bet on soccer in the last 10mins if you want when one team is a long way ahead. Then withdraw it. I bet on Man City to beat tottenham, won some oney then withdrew the £10 I initially put down.
2) same company, dif offer. Deposit £10 on vegas / casino. Vegas is transferable to sports, do same as 1.
3) same company make a £10 bet. I bet on Uruguay to beat Poland. Uruguay were 3-1 up with 8 mins left.
You can withdraw the full amount (no fees charged) and costs you next to nothing. In my case I actually won a little.
I got 8100 credits for that. For free.