Balsa can soak up adhesive and the glue is heavier than the wood. Normally it would be used as a stiffener in a composite, for example glass fibre where it would be placed between layers of mat or cloth. In that application it works well giving great rigidity.
Excellent material. It needs to be integrated into the design properly. Sufficient design data exists in ANC-18. We use this as an alternative in several modern composite aircraft designs. Beware moisture ingress and absorbtion.