Okay take this for an example. I buy 6 x 600 EURO bonuses on sweet bonanza, I get 9 dead spins on 4 out of the 6 games. I lose my entire 5000 EURO balance. How does that not sound shady? Not a single decent win on 6 bonus buys for 600 EURO? When I purchase a 750 EURO bonus on wolf legend megaways I win 11 EURO back on 18 spins lol.
Ignore the currency value for a minute, lets just go through each example and do the maths on the probability of each outcome. That would be the best estimation of whether what is happening most likely to be legit or not.
Go to a real casino or possibly any operation and they watch the probability of lots of games, when something close to impossible or highly unlikely occurs they are stood up staring at that guy to check he isnt pulling a trick. Same for users, its a good to keep a rough idea of how likely am I to win on this.
The easiest thing to do of course is stick to games with close to 50/50 odds like the red/black on a roulette wheel. Its not perfectly even but you have a rough idea you should win out of 6 rolls of the wheel.
I will find a website with a calculator to help with this hopefully.
Ok so this is a good example, they showed the example of always winning. But the reverse would also be after not many spins it does become unlikely to always bet the wrong colour on a roulette wheel
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A calculator to use on this one
http://www.casinoguardian.co.uk/roulette/betting-calculator/
What can happen when the odds are wrong:
https://www.rouletteonline.net/biased-roulette-wheel/