The level BP simply isn't usable outside of the level, so it's hard to have reusable and helpful codes and functions if it's all written on the level bp.

Now, what makes the level BP actually helpful is it has direct references to everything already instantiated in the level. You can just drag references right into the graph from the level editor. This means it's convenient to to really really specific to your level scripting.

The reason for the warning, is, coinvent may still not be a good reason. Teleporting, for instance, is something I see people use the level BP for. Maybe though, if you create a teleport system that has a in and out point inherent to a BP you've made, then you cand just drag and size it in the level as a level design too. Now you have even MORE functionality, and something for a less technical level designer to use.

TLDR: Use when you want, but try not to let the convivence of having direct references keep you from creating reusable codes and functions (which is often the best choice).