Aside single instances like Witcher 3 where there was some confusion as to where GMG purchased keys from, they are known as a legitimate site. I have no reason to suspect that their would be any funny business with the Max keys, as WB is highlighted at the bottom of the page as a publisher partner.
Other key sites are something that I really don't take a hard-line stance on. People are looking for deals, and most of those keys work, but I'm very skeptical of where they were acquired from in general. My stance is generally more neutral. They're not illegal. You're paying money. But I don't like that you don't know where the key came from and there's no guarantee that it will work in general (one of the sites that we actually have banned offers an optional, additional fee to guarantee a working key just for that reason).
So, the short of it is that they're not something I encourage, but also not something where I'm looking to ban people for using or anything. I personally got the game from GreenMan when they had a 50% off voucher for that game.