In my opinion, Kakuzu is still the best round 1 blitz team period (based on how quickly he deletes a team without the time delay of a mystery). However why most people would not consider him op is the rise of newer ninja that take away what makes him a blitz: his standards and/or his buffs.

Rinne-Sasuke is usually paired with LM and Itachi so they have a res buff to survive 1 standard (with Itachi randomly dodging). On top of that, there is a 30-50% chance to cause fear which stops him cold regardless of immune. Combine that with the mystery that gets rid of his Superarmor, a crit reducing chase, and the most elite scaling passive in the game so far means that if you can't kill at least 1 unit by the end of round 1 you are probably going to lose.

Halloween Kimimaro who is now in contention for best blitz unit will also be paired with LM and if you aren't faster than him will absolutely ruin your biggest damage dealer by reducing your standard and chase damage by 30% while also passively dodging both standards anyway. on top of that, his mystery only costs 20 so if he gets it off you can bet Edo-Hiruzen (his dynamic duo in most cases) is right there with a now 60% buffed attack.

Edo-Minato (who is unofficially the most meta unit in the game now) has 3 different effects on him that make Kakuzu all but useless. If he isn't immune then his enfeeble reduces his ninjutsu damage by 25% while also disabling his scaling (not sure if chakra scaling is fully affected). On top of that Minato will also have a passive that lets him dodge 50% (read: 80% really) of all standards coming his way. And if you are facing against the true elites he will also have a barrier that boosts his def/res while also canceling chakra gain. While he himself doesn't scale crazy damage-wise his sustain by itself will let him win against Kakuzu.

Edo-Itachi was basically created to be a Kakuzu stopper with his percentage damage reduction passive. (-50% nin damage for the first 2 attacks sounds particularly specific). His dream mystery is also very deadly as he can simply hit Kakuzu and take away all his buffs and immune at the very least.

Edo-Madara is not seen as much but when he is there it is basically a loss (most Kakuzu teams do not include a proc barrier after all). Getting no chakra for 2 straight rounds while also being unable to damage him at all (nin-immunity) unless you target him specifically for a mystery means that you are focused on trying to kill him with no scaling whatsoever while your opponent has free range to actually run his gameplan.

Yes, there are support ninjas that are gimmicky enough to cause problems (i.e. Summer Kurenai and Ibiki) it is really this main 5 make Kakuzu less of an auto and win type of guy and require finesse both on the mystery usage and the actual team construction.