Gravitational compact astrophysical objects are excellent laboratories to test the strong field regime of theories of gravity. Among these compact objects, lies the ultracompact class: stellar structures that possess a light ring (circular null geodesic). Such ultracompact stars were presented in literature in the earlier solutions of general relativity, and some are claimed to be good candidates to the supermassive objects present at the center of galaxies. In this paper, we present evidences for the claim that compact objects with a light ring should be black holes, based on the existence of long-lived modes obtained through a first-order perturbation theory. These first-order long-lived modes can source nonlinear terms which could turn the star unstable. We show, in particular, a comparison between modes computed through an exact direct integration and through the WKB approximation. Moreover, we present the time evolution of wavepackets for different field configurations. We conjecture some possible outcomes of the nonlinear instability. The discussion presented in this work complements our previous paper [Phys. Rev. D 90 (2014) 044069].