.onion addresses are just 80 bits, not 256. Someone estimated[1] that the amount of work to brute force one address is approximately the same as the amount of hashing that has been computed by the bitcoin network up to now, so it's clearly not physically impossible. This weakness is apparently a practical problem, because it made people unwilling to automatically issue SSL certificates for .onion addresses [2].

[1] https://news.ycombinator.com/item?id=11388997 [2] https://blog.cloudflare.com/the-trouble-with-tor/