  Adaptive filters exploiting sparsity have been a very active research field,
among which the algorithms that follow the "proportional-update principle", the
so-called proportionate-type algorithms, are very popular. Indeed, there are
hundreds of works on proportionate-type algorithms and, therefore, their
advantages are widely known. This paper addresses the unexplored drawbacks and
limitations of using proportional updates and their practical impacts. Our
findings include the theoretical justification for the poor performance of
these algorithms in several sparse scenarios, and also when dealing with
non-stationary and compressible systems. Simulation results corroborating the
theory are presented.
