PC How legit is G2A?

I personally bought around 20~ games from g2a and i had a problem only one time, the key resulted invalid. I contacted the support as i had the g2a shield thing and they provided me a new key free of charge, it was a bit of an hassle because they wanted screenshots and took a while to respond, but at the end of the day you always get what you paid for.

However i do not know exactly how things works when you get an invalid key and you don't have g2a shield, can someone please tell his experience in a situation like this?