Like someone else mentioned, there's no need for Node pointers at all here, storing Nodes by value cuts away a decent chunk of complexity and should run plenty faster (especially when threads are linked).

I would consider switching the unordered_map to a sorted vector<pair<T, Node>>; or even better, move the key into Node and simply store as vector<Node>. Hash tables are relatively expensive to create in exchange for excellent performance for large tables, and you're creating a lot of tiny tables. A regular map would likely fall somewhere in between performance-wise since they're cheaper to create.