Granular flows exhibit a strong dependence on the solids volume fraction in addition to the deformation rate. In the steady inertial regime these dependencies lead to the μ(I),Φ(I)-rheology. Here I first show how this model captures key features of the steady flow of grains in vertical pipes - despite this being a complex inhomogenous flow crossing regimes. I then highlight the major shortcoming of the I-Phi paradigm: ill-posedness in transient flow at high packing fractions. Thankfully, this obstacle may be overcome by including the correct dependence on the rate of compression/dilation, which is only nonzero for transient flows.