Two major reason why I don't use glass beads for weighted blankets and weighted lap pads: Cost & Construction.
While glass beads for weighted blankets have the benefit of being dense, they are very costly. Glass beads tend to be 2-3x more expensive than Polypropylene pellets (poly pellets).
Construction of a weighted blanket with glass beads can be a little tricky. Glass beads are similar to the consistency of sugar which means they may pass through some fabrics, as well as transfer between pockets in the blanket. Lining the fabric with a muslin/cotton and reinforcing the stitching is recommended.
Bottom line, there is no real benefits to using glass beads for a weighted blanket unless you are limited on space.