Son's tally last season was a massive outlier, but putting that aside, outscoring xg is only one aspect of being a good player. Granted, Son at his best is REALLY clinical, which is why he's still a top player.
Outside of scoring Son is severely lacking in many important areas. He's weak, he can't retain possession, his first touch is inconsistent, can't head a ball, doesn't press or track back well, doesn't have any vision to speak of, can't win the ball in a tackle, etc etc.
Apart from goal scoring Kulu is the better winger because he has a more well rounded game and influences the game over 90 mins.
Bentancur is better than Son for the same reason. Whether he can keep it up is a different matter.
27 g/a season before last. 21 before that. That was no outlier. Unbelievable at getting in behind, brilliant finisher. You may not have enjoyed watching him but he terrified defences, scored so many big game goals & was a top 5 LW in world football.
Bentancur/Kulu have so, so far to go to get to prime Son level. Are they silkier to watch? Yes. Are they more effective? No, absolutely not.