Ace was the 2nd division commander of the Whitebeard crew, but he hadn't displayed power befitting of his title. Besides the fact that he was a Logia user with the power to control fire from Mera-Mera No Mi, he has not demonstrated any other type of powers like Haki.
In the Alabasta Arc, he fought against Smoker, but he wasn't able to put up a good fight, claiming that both were Logia users and the fight would end with no result.
This also raises questions about Smoker's Haki abilities and the fact that even Smoker wasn't able to use Haki at that time.
- He wasn't able to hit Blackbeard when Blackbeard used his Logia power.
- He showed no resistance when Akainu hit him, not even weak Haki, and took the hit head-on.
He never fought Logia users with Haki, nor has he shown remarkable Kenbunshoku Haki.
Haki was revealed after the time skip, but it has been there since Rayleigh's and Roger's times. Were Ace's abilities limited to his Devil Fruit power only?