as you already mentioned, reachable extraction forces strongly depend on the kind of inserts and on the density of the foam as well as on the processing conditions. Our member EJOT, who is providing several kinds of inserts for EPP, provides detailed information on axial extraction forces for various foam densities in their technical data sheets. You can download this product related literature here:
My experience is, that the given values are rather a lower boundary and are normally exceeded when applying correct processing conditions. We also know that tailored solutions can lead to quite higher bonding forces, but for this it would be relevant to know the specific application / parts.