Of course ferrite distributed magnetic particles represents an effective air gap, but the quality of such ferrite varies greatly with type of ferite slurry and quality of supplier process. In effect the higher mu concentrations have more variability and minute flaws in mu can cause localized saturation which reduces inductance and when temperature rises can lead o a thermal runaway, which is why Flybacks are often limited to 100W unless additional larger air gapped cores are used to linearize the response. For larger powers , asymmetric forward half bridge converters are far superior.