I thought that the developers' concerns were just with sites like G2A and Kinguin who are notorious for allowing the sale of stolen keys which cost the developers money. I avoid such sites anyway but still keep an eye on CD Keys if I'm looking for something cheap at launch when I do decide to buy a new game.
I've used CD Keys a couple of times-my understanding is they bulk buy at cheaper prices-either from direct deals with the publishers or in territories where the game is cheaper. That doesn't seem a bad business model to me whereas the likes of G2A are pretty knowingly, despite what they say, positioning themselves as a shopfront for dodgy stolen keys.
I only buy from Steam, GoG or Greenman gaming (GMG) and very occasionally from Humble store.
I tend to buy from Steam or GoG as their returns policy means I can get a refund if I can't physically manage to play a game - either the control scheme is unsuited to my limited physical ability or there's other issues preventing me from playing. If I bought from a key reseller I'd have wasted my money.