In many molecular systems one encounters the situation where electronic excitations couple to a quasi-continuum of phonon modes. The interaction to that often structured continuum may be highly frequency dependent, e.g. due to some weakly damped high frequency modes. To handle such a situation, an approach combining the non-markovian quantum state diffusion description of open quantum systems with an efficient but abstract approximation was recently applied to calculate energy transfer and absorption spectra of molecular aggregates [J. Roden, A. Eisfeld, W. Wolff, W. T. Strunz, Phys. Rev. Lett. 103, 058301 (2009)]. To explore the validity of the used approximation for such complicated systems, in the present work we compare the calculated (approximative) absorption spectra with exact results. These are obtained from the method of pseudomodes, which we show to be capable of determining the exact spectra for small aggregates and a few pseudomodes. It turns out that in the cases considered, the results of the two approaches mostly agree quite well. The advantages and disadvantages of the two approaches are discussed.