there's no guaranteed winning when it comes to gambling. most of these bettors who are offering services with very high chance of winning are fake. you are right, if they can earn such money, why ask other people to avail their services?
Nevertheless, you're wrong when you said they provide a form of refund.
When it happens that they get to bring you a prediction and requires you to bet it on the terms of a 50:50 share on profit or
Instances where one don't get to bet the game they are offering you
That's got yo be some huge scam for a prediction.
It's a typical trier and error situation and your better off making your own predictions with what ever statistics you've got.
Should you hope to get predictions from sites and groups, be sure to meet some really false predictions and do your own scrutiny as well on them predictions.