"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 townspeople 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 Auger comments that even without his fruit his physical abilities are impressive as expected of a Whitebeard commander. Fought off jinbe for 5 DAYS who 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 been12 hours or so.
"Ace was Devil fruit reliant as a result 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 Kings 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 he had haki why didn't he use it against Blackbeard" Who's to say he didn't? 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 fruit and BBs 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 he was not 100%.
Other Ace feats:
- Clashing with an Admirals (Aokiji) and stalemating
- 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 on the Pirate Kings ship, the government wanted to use his reputation to scare off pirates, Ace had no reputation as a rookie he got it purely because of strength.
- His haki was praised by shanks and even Shanks said he was strong (when a Yonko praises someone that's when you know this person is extraordinary)
So guys cut Ace some slack he has some amazing feats and hype here he just didn't get much screen time. Oda said in this wano arc right now he will expand on Aces backstory and probably see more of his move set and haki abilities, but 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 there's a reason why Ace had to be killed off, it's because he's much more talented than luffy he would have become another pirate King if he reached his potential (whitebeard would have made him)
nikka just sliced a whole government building in half
