This work reports a detailed study of quasi-guided mode resonances in metallodielectric photonic crystal slabs, focusing on the interaction with the intersubband transition supported by a multiple quantum well embedded in the photonic crystal itself. The strong light-matter coupling regime is obtained at room temperature in the mid-infrared spectral range, and its spectral fingerprints are analyzed in the framework of a Fano line-shape model. We propose an estimate of the light-matter coupling coefficient for arbitrarily shaped resonant cavities embedding multiple quantum wells supporting intersubband transitions, obtaining good agreement between theoretical calculations and experimental results.