Is Null's Brawl Safe for My Phone? Debating Downloading

So I'm seeing a lot about Null's Brawl and it looks tempting, especially with the unlimited gems and gold. But I'm a little worried about downloading stuff from unknown sources. Is Null's Brawl safe for my phone? Have any of you downloaded it before? Were there any viruses or malware? I'd love to try it out but don't want to mess up my phone!