I discussed this in the race thread, but it seems to me that the issue of Ferrari's competitiveness in Monza (and Baku) has a lot to do with their low df package not being strong (and their high df package is very strong).
What highlights this the most is how, in both races, RB were comparatively good. Horner backs this up in the sense of illustrating that the RB car has a great low df package (Ferrari's being poor by comparison):
"Horner said he does not believe that its initial low-drag idea helped it at Monza, which he instead credited to work that had done on a specific low-downforce configuration."
https://www.motorsport.com/f1/news/horn ... ne-949318/
Edit: Of course these 'packages' are related to the characteristics of the car; it could be that, for example, the Ferrari side pod solution is not efficient downforce or that the Red Bull solution works with more emphasis on the diffuser, which we know is very efficient. Likewise, obviously if you have the best engine, it allows you to do things others can't - e.g. Both Mercedes and Ferrari ran a spoon rear wing in Spa but only Mercedes could retain it in Monza, with Ferrari needing something more closely resembling a 'tea try' (I.e. Flat).