Denial of service attacks are effective against centralized servers because only a small amount of servers are responsible for providing a service for their users. These servers can be flooded with requests from a large amount of computers/bots, making it impossible for them to do their job.
On a decentralized peer-to-peer network, instead of a small amount of servers, you have a large amount of nodes doing this job. The bigger the decentralized network, the more uneconomical the DDoS attack would be. A perfect example of this is Bitcoin, since it's inception it hasn't experienced a single network outage. To even further prevent a DDoS attack, the IP address and port of Streembit nodes are encrypted.
Centralized server vulnerabilities:
With central servers, you never know what you are going to get. Do they encrypt your data that they store? Are they up to date on the latest security standards? Do their employees be careful when giving up user information, and watch out for social engineering attacks?
Fortunately with Streembit none of these problems will concern you, your data isn't stored on a server and due to the nature of public key cryptography nobody even needs to know what your password (private key) is in order to verify that you are you.