In the case of SAFE, you can generate as many nodes as you want, but they must be valid nodes (doing the work that the network asks of them) for a long time before being promoted to a status that allows them to participate in consensus of a section. Any misbehavior on their part results in demotion or failure to be promoted. So each node would need to be a legitimate contributor to the network, with all that that entails, before being promoted to a status where their later misbehavior would matter. You would be creating real nodes, that used real bandwidth and real CPU resources and did real work that benefits the network, in order to infiltrate your evil nodes into the network. So, not an insubstantial cost to you.

Your nodes will be assigned into sections randomly, so it will be very difficult for you to manage to get multiple nodes with voting status, into a single section, and staggeringly difficult to get enough such that your nodes constitute 1/3rd of that section. Of course, exactly how difficult depends on network size (number of sections).

Finally, even once a node is assigned into a particular section, it will be reassigned to another random section at some interval, further decreasing your ability to take over a particular section.