Beryllium has several GIANT disadvantages.
The main one to stop every normal person is that it is poisenous,easely into lethality. Thats the main reason it is outlawed in Battlebots, not the pricetag-the next disadvantage-

And main thing, if you try to buy enough Beryllium, or certain alloys of it, you can expect a visit from the Man In Black-its a main component for any effici«nt small nuke or hydrogen bomb.