Wise Feline Utility AI for Unreal
Wise Feline is a utility AI system which allows you to make immersive AI with ease. It works great for agents in complex situations and has a great visual debugger.
Behavior trees are more managable than FSMs (Finite state machines) but they can become easily very complex too and when they have to handle complex behaviors, the trees become almost impossible to read and understand. They also depend on the order of nodes which makes them even harder to understand and modify.
Finite state machines, while being the oldest approach used have their own drawbacks too. You need to explicitly create all state transitions and you have to anticipate all things which can happen. Their complexity can easily go out of hand too.