Is Homebrew safe for Mac?
Homebrew on its own acts like a command-line App Store. 


It's safe, if you know what you're downloading.
It uses SHA256 to fingerprint the downloaded instructions as a validity / tamper detection verification check. 
It’s open, so you could validate what it’s downloading and how it
works.
It refuses to use sudo to intentionally be safer. 


If you are concerned about downloading hijacked binaries, verify the SHA/SHA1 sum of the binary you've downloaded against the SHA/SHA1 sum published by the developer, usually on their webpage.

Despite a long track record of people not pushing malware or bad code to the system, it would be good hygiene to start validating downloads.