For me honestly it's Kaido's invincibility
The idea that Kaido has been "unscarred" except for Oden makes ZERO SENSE
Captured by Navy
Captured by Yonko
Tortured countless times
sentenced to death 40 times
Was in Rocks pirates, and faced countless legends who knew Ryuu better than Oden
Yet, somehow, only Oden was able to scar Kaido, and basically, we have clue that the scar was caused by both Ame no Habakiri and Enma, so it is NOT ONLY because of a sword, it's simply Oden's Ryuu, and trying to justify the countless times of being captured and sentenced to death by Yonko and Navy makes NO SENSE
the invincibility plot of Kaido just feels like a huuuuge failure of a plot by Oda...
And EVEN NOW, for some reason Oda wants us to be convinced that Kidd attack is affecting Kaido when it's blunt raw force attack with NO HAKI at all.... not even normal armament cladding, just wrestling moves that freaking Franky can do BETTER....
Oda started to make Suplex affect Kaido... when basically Gear 4th with cladded haki before didn't drop a single blood from him
this whole thing is a HUGE Plot Hole and a failure by Oda in my book... and it's HARD to correct it by Oda... cause he shouldn't have made those comments about Kaido in his introduction
The idea that Kaido has been "unscarred" except for Oden makes ZERO SENSE
Captured by Navy
Captured by Yonko
Tortured countless times
sentenced to death 40 times
Was in Rocks pirates, and faced countless legends who knew Ryuu better than Oden
Yet, somehow, only Oden was able to scar Kaido, and basically, we have clue that the scar was caused by both Ame no Habakiri and Enma, so it is NOT ONLY because of a sword, it's simply Oden's Ryuu, and trying to justify the countless times of being captured and sentenced to death by Yonko and Navy makes NO SENSE
the invincibility plot of Kaido just feels like a huuuuge failure of a plot by Oda...
And EVEN NOW, for some reason Oda wants us to be convinced that Kidd attack is affecting Kaido when it's blunt raw force attack with NO HAKI at all.... not even normal armament cladding, just wrestling moves that freaking Franky can do BETTER....
Oda started to make Suplex affect Kaido... when basically Gear 4th with cladded haki before didn't drop a single blood from him
this whole thing is a HUGE Plot Hole and a failure by Oda in my book... and it's HARD to correct it by Oda... cause he shouldn't have made those comments about Kaido in his introduction

Why should this be a plot hole ? Mangas would be filled with plot holes if this is one.