I think the reason why folks are pushing back is that to date, Feige has run a very tight ship in terms of continuity. There's certainly been some kerfuffles along the way, such as the "8 years later" error in Homecoming, but most of what's been onscreen has fit within a clean narrative.
When you start plucking characters from other cinematic realities the continuity can get quite messy. Is Foxy Pete, who was an INO character in the Kinberg-verse, now the character we recognize from the comics? Is the First Class series, which most of us were happy to see ended, now canon? As Deadpool once said, which version of Professor X are we going with?
Maybe Feige and his team can pull it off but he may be adding some unnecessary comic-book complexity to what has been a clean narrative throughline over more than a decade. So for now I am hoping that Peters is a guy posing as MCU Pietro and not the Fox Version. We shall see.