When you install Bitcoin client, how does it discover the nodes to get a copy of the blockchain? Are their seed nodes ? Are they vulnérables? SPOF ?
There are several hard-coded DNS seeds, which work AFAIK as a DNS round-robin to provide connection to several random nodes.
Once initially connected to nodes, peer discovery is automatic and your node could easily learn about the entire network through just a few connected nodes.
There is a slight chance that a malicious ISP could hijack DNS or for that matter, all Bitcoin traffic, either preventing peer discovery or alternatively providing false or limited results. This has already been asked elsewhere. Provided you are able to connect to just a few well-connected nodes on main-net everything will work as expected.