I dabbled with Sketchup some, and you could easily make a wide variety of structures, and it has many plugins and finished model sets.
I was wondering how problematic would it be to use Sketchup as a main architectural tool (from huts to mansions) to import into UE4?

I guess not.

This would do better in the art forum. Post in gamedev.net can take up to 24 hours to be responded to.

Sketchup can be used in games, you just need to clean it up first.

You can download Blender to use along with Sketchup and use Blender to clean the models. That way you can get the easy to use interface of Sketchup  and the powerful modeling tools of Blender. You will also need to clean the UV maps as the models in Sketchup have a per poly uv map that will make the models more expensive to render.

In the end Blender would be a much better, as it makes clean models and does it a hundred times faster.

Blender is the 3rd most powerful all-round 3d modeling tool, just after 3DsMax and Maya. It's also free where the other two is more expensive. It has a hard to learn interface but is worth the time.

Just now, Scouting Ninja said:

Sketchup can be used in games, you just need to clean it up first.

Ah, that's a no then. I don't need any more extra fiddling work.

I just thought that I could use it instead of Maya for this specific purpose, but I guess I'll just use Maya for everything, given that I already have a 3 year license.

5 minutes ago, Armantium said:

but I guess I'll just use Maya for everything, given that I already have a 3 year license.

You should always test your choices of 3D tools before buying one and only after learning.

With Maya bought, paying for any other tool except a 3D sculpting tool is a waste. Maya is a 3D all-round tool, it can make any game model from start to end. Also once you have learned to use it, it's going to take 3 years for game models, you will no longer ever consider using a tool as limited as Sketchup.

Have fun learning 3D, it's a really interesting field.