My main problem with these fan made matchups, between characters that were introduced in different parts of the journey, is that Oda keeps introducing new concepts to the story and we have powerful enemies falling along the way, so they later become laughting stocks for the fanbase with Crocodile being the most obvious. For example, lets say One Piece can be divided into 3 parts, early OP, inbetween (powerups to make the SHPs keep up, but no haki still) and Post TS haki haven. At 1st, it was supposed to be Luffy and his crew searching for Rogers treasure, while beating up bad guys that threaten his friends on the way. Then Robin and the poneglyphs came into the story, so now you have the government and its dark behind the scene schemes in your way, plus now its not the shichibukai that are the scariest, but 4 super pirates were revealed. We already knew about the shichibukai and Luffy tapped into that teritory during Alabasta, but we also saw Ace, and i could not grasp how someone as powerful as this guy is only the 2nd division commander for Whitebeard. Then Buggy told us that Whitebeard was Rogers rival so OK, it was understandable. But then Garp tells us hes only one of 4 such monsters...
But back to Luffy, so he beats Crocodile and Oda realized since he will extend the story further that the carrot he made Luffy chase is too close, and Luffy can stretch his arms and just take it, so he needs to further extend the gap, so far away that Snakemans Python couldnt even reach it. He expanded Luffys powers during EL so he could keep up with upcoming enemies, even though one of them was also a shichibukai like Crocodile. Then at Sabaody, they looked helpless against a human weapon built after (aka being a cheap ripoff copy of) another shichibukai. Then the timeskip happens, and Oda expanded on the idea he introduced just recently, haki. He invented hardening, he added FS later, he expanded on Rayleighs display of advanced CoA. And then if you look at the MF war, no one has shown any of this stuff, and earlier in the story no one even displayed the basics of it. Its Odas way of furthering the distance between the end goal and Luffy, and giving him stuff he can improve on to justify him reaching that end goal, but in this scenario characters like Crocodile, Enel, Moria look like fodder to even Doflamingo, if you just read the fights and have no clue whos who. After the timeskip Luffy causally one shots a pacifista, giving the impression that post TS Luffy >>>>> pacifista >>>>> pre TS Luffy (a stronger version of the guy that defeated Crocodile). Then in the Doflamingo fight Oda further widens the gap to pre TS, when he showed that a so much improved Luffy with haki couldnt even make Doffy say 'oh, that one hurt' (remember all of Luffys feats of strength and power pre TS, then use pacifista as measurement and see how much he improved, yet Doffy called him flat out weak), until G4 made Doffy jump around like a ping pong ball, giving the impression that G4 >>>>> Doffy >>>>> post TS Luffy, which would be fine if Doffy wasnt just another shichibukai like Croco. So is Doflamingo really THAT much stronger than Crocodile, a guy who challenged WB in the past and who clashed with fellow shichibukai, yonko commanders, an admiral and Doflamingo himself during MF? Ofcourse not, im sure if haki and gears were a thing in Alabasta, Oda would make Luffy use them and win with same difficulty, but they werent at the time so Oda just used the resources he had available to eliminate a threat that had to be taken out. Or so to say, Luffy (or any character really) is always as strong as Oda needs them to be.
Or in short: