I think clam fight the DDoS attack just like bitcoin does it. To attack blockchain with a DDoS attack is a waste of time because you can take down 1 node, or maybe 100 nodes, but not the full network.

About the 51% attack, that could be done at any time by the casino Just-Dice because they are the main holders and miners. SO, if someone make that attack at least we will know who was behind it. A bit with time JD has proven to be a casino we can trust.