How easy is this to use for a beginner? What are its limits? I assume a full open world game would be impossible
I'd say the plugin requires intermediate experience with construct and 3D tools. It's very easy to make simple things, even for a beginner, but more complicated 3D games require an understanding of trigonometry / linear algebra, and basic 3D graphics principles. The plugin is much simpler to use than it used to be, but it's not meant to be extremely beginner friendly as this would limit the flexibility / speed of the 3D engine a bit.
All in all you need to expect that Q3D doesn't do the work of building your game for you, but gives you some nice tools to work on 3D web games / applications using construct.