I mean Kidd and Law both use Haki and have gotten awakened DF powers in order to defeat Big Mom, but both deciding they were worthy enough to go solo other Yonkos is this biggest laughable mistake, yet we still have people here acting super shocked that Law and Kidd would job badly to Shanks or Blackbeard.
Luffy managed to get advanced armament Haki that allowed him to use emission control to reach another level and he has awakened the strongest mythical zoan in one piece in order to solo a Yonko that also has a mythical Zoan and a good degree of Haki. Zoro fought the strongest commander, Sanji the 2nd strongest commander and Jimbei basically fought the strongest flying 6 member who was confident enough to challenge and throw hands with Kaido's top 3 commanders, and there were some other strawhats that were fighting flying 6 members. KILLERS MAIN OPPONENT WASNT EVEN A FLYING 6 MEMBER his main opponent was just Hawikins!!!! and Law's meme panda doesn't seem to have any considerable feats that would put him around Killer's level. Even if Law or Kidd have the strongest DF and advanced haki levels like Lufffy. Killer and meme panda are so weak that it is even up for debate if they would offer a challenge to Brook, Usopp or Chopper(give them the benefit of a doubt they beat Nami) while Shanks, Luffy and Blackbeard have a bunch of members with memorable names and faces whom are showcased to be strong characters. If either Law or Kidd were on Luffy's level to fight Luffy solo wise they are going to get jumped by the other yonko crew members after they leave killer and meme pandas corpse behind.
Its like why is everyone shocked on this forum that Kidd or Law would offer any kind of challenge? To me neither will have any significant plot because they will need their own timeskip training and more strong crew member which by than with the current pace of events on one piece Luffy would be pirate king by then. So best case scenario would be if they serve themselves as underlings to the straw hats.