Our previous investigation showed that nanocrystalline silicon proved to be an interesting alternative to monocrystalline silicon for optomechanical crystal cavities, with similar or better performances in some cases, but at a much lower cost. In our recent work, we have deepened our investigation into the optical, mechanical and thermal properties of nanocrystalline silicon as a material platform for optomechanical crystal cavities. We have used a number of structural, optomechanical and thermal characterization techniques to evaluate the pros and cons of this platform when compared to monocrystalline silicon. We demonstrate that the microscopic structure, and in particular volume fraction of grain boundaries, which were tuned by changing the annealing temperature, strongly correlates with modification in optical, mechanical and thermal properties.