The major difficulty is the choice of models for dynamic viscosity, density, thermal conductivity, and others. These thermo-physical properties change with volume fraction, temperature, nanoparticles radius and others for a simple nanofluid and significantly when the nanoparticles are of two kinds.
Another important topic is the choice of either type I model or type II model of hybrid nanofluids. However, there is an insight in the report below:
Significance of
suction and dual stretching on the dynamics of various hybrid nanofluids: Comparative analysis between type I and type II models. Physica Scripta 95(9), 095205.