Doffy was strong enough to be granted the rank of Shichibukai even though the CDs despised his family for rejecting their god status.
He had CoC. The biggest indication for strength and potential that ww know of. If someone claimed that Doffys strength was not enough to get the title then I can only take this seriously if there is a good explanation as to why.
Its easy to explain for Loki. It took a Yonko to get him back under control. Loki is obviously a beast.
Doffy was trying to avoid conflict with an Admiral if possible, but was ready to engage in combat with Fujitora if it became necessary. Its not a path someone would take if they are sure they would lose.
Latest villain syndrome.
Every arc the villains of previous arcs drop by 1-2 tiers in the perception of the audience.
Doffy was not chosen because the CDs are petty cunts. Also Loki is the heir to the throne of Elbaph. Its easier to explain why god status was granted to a king than to a well known commoner like Doffy. It would raise questions when Doffy was picked for the job, but guys like Mihawk were not. For Loki no such political problem would arise.
Consider Doffy chose to kill Smoker, knowing that if he was able to do so, his fight against Aokiji was inevitable. As he failed, there was no reason to fight Aokiji, all this after he gave Aokiji a free shot which failed to fullly stop Doffy. That same Aokiji literally told Smoker to tell Akainu to use admirals to check the matter.
Consider also, that Doffy was allied with Kaido and were preparing for a war, Kaido chose Doffy over people like Big Mom lol, and at end, it was a Yonko who replaced him as partner for Kaido.
By words of Caesar Clown, not once, but twice, he said that he was save under the sleeves of Doflamingo against Big Mom. Such was his confidence to Doflamingo, he knew Big Mom wouldn't go after him.
Luffy after defeating Doflamingo, Fujitora mentioned that now his new threat were gonna be Yonko, not commanders.
Oda mentioned that Doffy was gonna appear at wano together with Kaido as a big threat, yet he chose to give him his own arc.
Took 2 supernovas to defeat Doflamingo, the same number it took Big Mom, the difference is that they had awakening, Big Mom never gave to Law the same level of damage Doffy gave him lol.
Doffy was the king of the underworld, the most confident.
Garp was after Roger, failed to capture him.
Sengoku was after Shiki, failed to capture him.
Tsure was after Doffy, failed to capture him and even had to accept his warlord status lol.
Doffy has a lot of potrayal and hype.