Happens all the time on dark net markets.
You should never leave coins on a site where you don't control the keys.
So would better if we move investment money periodically from a site to another site.
Who knows a high reputation site like betking.io with over 3,000 bitcoin invested can run away but we wish it will never happened.
As already said by BTCevo the scam can take place at any moment, maybe you invest today and the scam happens tomorrow, bad luck you but possible.
Also, if you keep moving your funds, you won't have the advantage of long-term earnings, hence you would be playing a riskier game (of investing just before someone wins big money, for example).