<p>It is challenging to analyze nonlinear dynamics of infinite-dimensional continuous structures due to mutual interactions among different modes. Single-mode approximation has long been assumed valid to approximate primary resonant dynamics of cubic-only (nonlinear) structures, although it is known to possibly fail for structures with both quadratic and cubic terms. This paper revisits this conclusion by focusing on a cubic-only structure with two competing dynamics, i.e., hardening (H) dynamics with frequency response curve (FRC) bending rightward and softening (S) dynamics with FRC bending leftward. Explicitly, for a cubic-only beam with both geometric and inertial nonlinearities, by comparing single-mode discretization perturbation with direct perturbation (with full-mode effect considered), it finds that: close to hardening-to-softening (H/S) transition, i.e., a critical state where hardening balances softening, the widely used single-mode approximation turns insufficient, i.e., giving enormous nonlinear prediction, while it is valid as commonly assumed when the system is away from H/S transition.</p>