Thanks for this, I will stick with GOG or steam.

Majority fraudulent? Possibly, but I doubt it. 99% revocation rate? No. They wouldn't have any customers left if that were the case.
G2A is no more shady than any other reseller that allows random people to sell stuff they got from somewhere else (this includes brick-and-mortar pawn shops and the like). They're all kinda shady; it's the nature of the business they're in. G2A has simply become the big dog in the 3rd-party vendor market for digital games, and thus get the most badmouthing. All such places at some point end up dealing in stolen goods (knowingly or unknowingly), and you either decide to deal with them anyway, or decide not to.

I wouldn't recommend that anyone patronize code resellers like G2A or Kinguin, but throwing around wild propaganda doesn't really help.

They're no more shady than Ebay. They're as shady as their resellers. I've bought quite a few keys on there and only had a problem with 1 key that I was refunded for.

Plenty of resellers are either people selling unwanted games from bundles or . There will be fraudsters using stolen credit cards but G2A have to refund anyone who claims against them so they'll quickly kick anyone off who is selling dodgy keys. The smarter fraudsters though will probably charge back on their credit card just withing the 6 months claim time after buying the keys.

The only dodgy thing is their sheild, because buyer protection doesn't need to be paid for.