Heterogeneous networks (HetNets) provide an effective way to meet the explosive growth of mobile data traffic. Previous studies have revealed that the successful transmission probability, i.e., the link reliability, of a SISO HetNet is invariant to the base station (BS) density. This indicates that the area spectral efficiency (ASE) can be increased by densifying the network, without sacrificing the link performance. However, in this paper, we shall show that the above invariance property no longer holds in multi-antenna HetNets. More specifically, changing the BS density will affect both the link reliability and the ASE, and there exists a tradeoff between these two important performance metrics. By adopting the Poisson point process to model the BS positions, we develop an exact expression of the successful transmission probability for general multiuser MIMO HetNets. We then use this result to evaluate the tradeoff between the ASE and the link reliability. It is analytically shown that the maximum successful transmission probability of the network is achieved by activating only the tier of BSs with the largest number of antennas per BS, while the maximum ASE is achieved by activating all the BSs. By adjusting the density of each tier of the HetNet, a tradeoff between the link reliability and ASE can be achieved.