Folia introduces regionised multithreading, which means that plugins will require modification to function properly. Plugin developers should expect compatibility with existing plugins to be at zero.
There are currently broken API, including the scoreboard API, and planned API changes, such as super aggressive thread checks, which may impact plugin development.
Your statements already answer your main question.
ItemsAdder will not support Folia as it breaks every Spigot plugin not only ItemsAdder.
I have no idea why Paper team decided to work on Folia, we already have Sponge as an alternative plugins loader for Minecraft servers and it's still not really used widely.
I don't know how they expect a new project to be popular.
Not here to throw shit at them since they surely worked hard on it, but I don't think many devs will consider switching or adding compatibility to Folia.
The regionized ticking is cool but idk if most established networks would also consider rewriting their WHOLE plugins ecosystem just to get some unknown performance improvement.
We'll see in the future if it's worth doing so, but I doubt.