As others have noted, the logic is designed as such that it's always possible. It's a graph theory problem, although I don't know if it's coded that way or just a giant pile of if/else.

One interesting thing to note is that some of them have difficulty settings. Certain item dependency chains are more difficult than others, and the locations can be done such that it will always give you easy seeds or always give you hard seeds. Some of them are designed such that you'll always be required to do speedrun tech which are borderline exploits.