DDoS

It can certainly help with it, as it will block IP addresses and also automatically trim the block list to keep it as small as possible. But if you wish to really stop D/DoS attacks, you’ll want to use something like CloudFlare. The ability of a plugin to handle such attacks is dependent on the capacity of the host. There isn’t a single WordPress plugin (no matter what they tell you) that can handle D/DoS attacks at any large scale as well as CloudFlare.