For this project I was simply trying to learn how to produce similar results to what I had done previously with addons using particles (Scatter, Graswald, Botaniq, etc). This type of scattering can be done with particles without addons, but I think it would be a pain. So one advantage of geonodes is simplicity when compared to particles without the help of addons.
In both cases I’ve had to drastically reduce the number of instances displayed in the viewport. Rendering millions of instances is no problem (Cycles) with particles or geonodes. But the performance of the UI goes downhill very quickly if you attempt to display too many in the viewport. This happens in any context, but perhaps it is a little more dangerous in geonodes as it is pretty easy to accidentally request far too many instances than the viewport can handle. Save your file very, very often when playing with any nodes that contribute to density calculations.
Not exercised here, but a major advantage of geonodes when it comes to scattering is the ability to have multiple ‘levels’ of scatter. For instance, I’ve tried to scatter moss on rocks and then scatter those rocks around a scene. With particles that was a PITA. Another example would be trees which use particles for twigs/leaves; if the trees are then instanced with particles, the leaves fall off. Geonodes should handle these cases with ease.
With particles, it is essential that the object you’re scattering on (usually a plane in these types of scenes) is subdivided enough that the displacement isn’t too far removed from the final results of the microdisplacement of the adaptive subdivision on the terrain. Otherwise you’ll end up with floating instances. Similarly, the vertex resolution needs to be fine enough that you can affect the scatter density to your liking.
A main point of this exercise was to see if this could be avoided with geonodes. It can, and I suspect it offers the possibility of higher precision placement if required.
Those thoughts are what comes to mind regarding simple scattering. Geometry nodes offers way more, of course. See @Gemn 's work to see what is possible.