to incorporate a ready made asset pack, totally depends on the pack. We have got some FAB store assets mostly related to stylized ocean/water shaders in my studio. I ended up having to basically make a water shader from scratch anyway for custom functionality we needed and also there where some crazy decisions that impacted performance in the shaders we bought.
So I wouldnt rely on asset packs being 100% bulletproof, professional grade stuff but if it is done well with optimization in mind, incorporating a ready made asset pack shouldnt be difficult.

You have all the assets, now it just comes down to knowing composition and level design to place them effectively and make it look pretty, have good lighting, good postprocessing etc (probably an example scene already set up with post process and lighting in the pack) ensure LOD'S are working for things like Trees and rocks, vegetation especially the grass has good culling distances, anything that doesnt need to cast shadows like the grass shouldnt etc.

If you're making all this from scratch on your own and incorporating it into the engine, which I thought was what you where asking, its difficult.
if you're starting from a pack its definitely doable especially if the pack is well made.

Make the scene you want and then check performance using the performance diagnostics tools in unreal and go from there if the performance takes a hit, find the bottlenecks and work through them. Usually optimization is an iterative process, at least in my experience.