By fully exploiting the spatial resources, unmanned aerial vehicles (UAVs) are expected to serve as an efficient complementary to terrestrial wireless communication system to provide enhanced coverage and reliable connectivity to ground users. With the growing deployment of units such as small cell base stations (BSs), however, the incurred severe interference may hinder the potential benefits of the integration of UAVs. In this article, we first discuss the intrinsic features and potential benefits of UAVs and introduce the architecture of multi-layer heterogeneous wireless network (MHetNet), in which traditional wireless network is assisted by UAVs. Then, an explicit discussion on the factors that limit the performance of MHet Net is presented, including the UAV topology, UAV density, and spectrum sharing of UAV and terrestrial networks. We use simulation results to investigate the performance of MHet Net in terms of spatial throughput (ST). It is shown that, together with the densities of UAV and terrestrial networks, the altitude of UAV is a limiting factor that should be optimized to improve the ST of MHetNet.