Hi, thank for your feedback.
So for your questions:
1. I use Quixel because, as for now, it's free and it have the Megascans gallery which are 8K resolution textures that can be used for 3D works so they have all the maps you may need such as Normal, Opacity, Specular, Roughness, Dispalcement etc...
And compared with other programs such as Substance Painter, it's by far more easy to create textures for large spaces such as environments  and buildings and also create procedural generated tiles for game maps.
2. As stated before, Substance Painter is slightly less good for large environments, BUT for specific 3D objects such as Characters and props Substance is superior as it is quit literally a 3D Photoshop program, in future posts i'll try to make a small comparison between both programs to give better understanding why 1 is better then the other in one aspect but less better in another.
3. Large spaces, procedural generation and of course ultra realistic resolution imagery, and also this post does show it can save you and your 3D artist a lot of time of modeling by simply applying a good mask and manipulating the textures to give the illusion of 3D.
4. The most challenging part of 3D for me is, still, working the on UV maps... it is so time consuming and difficult.