AI assets... yea or nay

That's really impossible for anyone else to answer - we don't know the requirements of your game, your programming experience, or your project budget or timeline.

If you're a skilled programmer, you'll always get better performance from a custom-built solution than a generic off-the-shelf. I've always written my own libraries for things like genetic algorithms and neural networks because I find they need to be tuned for each project. But I use generic libraries for steering behaviours and FSMs.

If you are interested in Behaviour Tree, have a look at Panda BT. It's a scripting framework, therefore it is intended for programmers. Also it is expected that you write your own code to implement your AIs ( Panda BT is a not collection of predefined AIs ready to use for beginners). Conversely the system is designed to support your own code (new or existing) easily. Of course, It works with Unity 5 and the latest version.

Panda BT works as advertised and comes with no bells and no whistles. I'm the author, so you are welcome if you have any question about using this package.