No bad writing. 
Just you still compare Pre TS and Post TS.
At Pre , we have small world . Just straw hat and his journey island by island. 
At Post , we entered very big world . There are lots of character that Oda have to meet us . Even Luffy hang out side characters more than straw hats.
Oda is giving story or arc for all SNs. Also , our main villain is more versatile and have huge connection anymore. to meet one yonkou and his/her crew is hard. 
As a result , any straw hat character did not take necessary time since post time skip. 
Cuz, world changed. 
And , sanji is not dirty . your expectations are very different.
In arabasta , sanji played Mr . Prince role and saved crew . No fight no another thing. 
Same Sanji made cake and saved crew . No fihgt no another thing. 
This is just one example.
People general say that vergo cracking , doflamingo destroy him , he cried , he hit his captain.
However ıf you ask  what happened on Manga  what oda want show us yourself , that is not hard to understand. 
In the new world , Oda meet us most of thing with Sanji.
In the fishman island , we did not see any haki abilities. real challenge started with Punk Hazard. Ceasar could not use haki. 
To meet how much haki strong is . oda need to show us . 
At he Punk Hazard , vergo was main villain.  
Over the Sanji , we saw real haki ability . But rest of arc , we saw sanji at everywhere and every point . sanji saved marine tashiqi and childs . he saved Kinemon. Half of punk hazard was saved by sanji . But some people obsessed with Sanji . and try to humilate over that case. like complex math problem , sanji was at good condition . sanji is like that vice versa . as if thet vergo is weak . 
During the punk hazard , just sanji and law faced real haki user. that is impressive to me. 
And we said oda made sanji dirty. This is one of examples. I can extend the list. 
ı hope you understand my point.
That is just one example .