“Ace didn’t master his devil fruit” Ace had a very good mastery of his fruit just look at the versatility of his attacks, also when Ace was on Drum Island the town’s people told Luffy’s crew it didn’t snow that entire day, which is strange because Drum Island is known for snowing every day, this hints at Ace having awakening abilities.
“Ace is physically weak” Van Augur comments that even without the Mera Mera no Mi devil fruit his physical abilities are impressive as expected of a Whitebeard Commander. Ace fought off Jinbe for 5 DAYS which is extremely physically overwhelming, showing Ace has incredible levels of stamina, only top tiers have been shown to fight for days e.g. Admirals, Shanks, Mihawk etc even Luffy’s longest battle has only been 12 hours or so.
“Ace was devil fruit reliant and his haki was weak” It’s been revealed through the Ace novels that Ace could use Armament Haki, which he awakened mid-fight against a vice admiral and it was STRONGER than the vice admiral’s own haki after just awakening it, there’s no telling how much stronger it would have gotten after 3 years of New World battles. We already knew he had Conqueror’s Haki, which he awakened as KID!! May I remind you when Luffy awakened it at 17 years old showing the difference in natural talent between Ace and Luffy. In fact his haki was so powerful in the Ace novel, when Ace steps onto the snowy Island where Shanks is, Shanks comments “his haki is like fire which is why the snow on the island is melting”.
“If Ace had haki why didn’t he use it against Blackbeard?” Who says he did not do it? Remember haki was always invisible pre-skip, the only way we knew someone used haki was if they faced logia or it was directly stated in the dialogue. Blackbeard is always tangible because of his Gura Gura no Mi and his crew evacuated the area so there was nobody else to comment on Ace using haki (also remember a lot of the fight was off-screen).
“Ace got fodderized by Akainu” Ace had been tortured repeatedly in Impel Down and was given little to no food or water so he was not 100%.
Other Ace’s feats:
–Offered title of Warlord at 17 years old! People will try to invalidate this by saying Buggy was offered it as well however Buggy got it because of his reputation of being linked to a Yonko and to the Pirate King, the Government wanted to use his reputation to scare off pirates, while Ace had no reputation as a rookie, he got it purely because of his strength.
–Clashing with Aokiji and stalemating
With all the feats I stated here it’s safe to say Ace is like “low tier Admiral level”, strong enough to halt and injure an Admiral but not beat one. That’s why Ace had to be killed off, it’s because he’s much more talented than Luffy and he would have become the Pirate King if he reached his potential (Whitebeard would have made him).
*Theory by Chari_D_Tbator