Beryllium is a good plasma facing material (low Z, low retention, low activation) and acts as a neutron multiplier, but it's highly toxic: only a few months ago ITER announced they scrapped the design of the first wall because working with beryllium was causing too many complications and slowing the project even more.

It's also so rare to be completely unsuitable for a power plant: a single DEMO-like reactor with a ceramic blanket (HCCB design) would require 70% of the world beryllium output to build and then burn through 200kg/year. Essentially you could only build a couple of these.