Hey WungielPolacz! The plugin itself should neither have too large of a performance footprint on your development nor should its use be encumbered much when using high-fidelity assets. What you have to take into account though, is that Godot itself (and other Engines as well) needs to be able to handle all assets that are used. If your scene would, for example, have a thousand realistic trees, that are just instanced as nodes (which the plugin does), you will eventually notice your speed tree to go down, and your frames drop, if you do not take any optimization measures (using LODs, editing with lower viewport resolution, etc.). Hence, if you place an asset that takes a few seconds to instance, than AssetPlacer will be as slow as Godot. If you had no problems using these high-fidelity assets in the quantities you need so far, AssetPlacer should handle them fine. One thing to note, however, is the size of libraries. The more assets, and the more complex they are, the longer loading their thumbnails will take. I recommend having no more than a hundred assets per library. As for surface placement, your objects have to include collisions, as of AssetPlacer 1.2.x.
Regarding the support and updates of the plugin, I can assure you that it will continue, and is in my professional plans for at least this and next year. You have to take into the account, that as of this year, I am still a university student, maintaining this project on the side, and I like to also do not-for-profit work, like contributing to the Engine (I recently worked on Godot's AnimationPlayer) and making free plugins.
I hope this clarifies your doubts, and I could maybe convince you to give AssetPlacer a try! If you have any more questions, please go ahead.
~ CookieBadger