DON’T ASSET BASH IF YOU ARE A BEGINEER

Asset bashing seems like an easy way to create games, you get assets for each aspect of the game, combine them, and voila, you have a game. This is a trap that most people seem to fall into when they start, including me. Taking different systems as a beginner, that are created in different ways with different performance and quality levels that need to be modified and stitched together, will just fail and you will most likely drop the project and not learn a lot.
The analogy for this would be taking different parts of the car and trying to combine them into a new car, and you started learning about cars a week ago. The end product would be a complete mess(if you somehow manage to finish it).

Combining multiple different assets is something I would not recommend even for intermediate users only advanced, who will know how to carefully analyze, edit and merge assets.

Disadvantages of asset bashing

The first problem that you have is the structure of the assets you are merging. If we look at data management; One could use data tables to store data and another could store them in variables. I had this exact problem on one project I worked on, where the guns from one asset pack used variables, and the inventory system used data tables. It was a nightmare to work with and everything short of completely restructuring one of the systems meant stitching two systems in some janky way.
Another is modularity, which could greatly influence the methodology the creator goes with when creating the asset. Modular assets usually have distinct objects that are independent of each other. This allows the developer to quickly switch things around and add his own content without dealing with a huge web of nodes and functions.
Yet another problem could be custom classes and code that become incompatible and have to be merged with the other asset.

And the last thing I should mention is performance, especially between two asset packs that are heavy with meshes and materials. There are a bunch of optimizations you can do with your assets, so you don’t have duplicate assets. One thing would be creating materials instances that inherit from master material or using textures like noise or gradient between multiple materials. If the creator of the asset pack you are using optimized it, the assets within the pack should work well with each other, but won’t be optimized with assets from another pack. This is additional work that you would have to do, so you don’t have tons of duplicate materials, textures, and meshes.
Ok, the last one I promise. Asset packs are usually made with the platform or use in mind(pc, mobile, cinematic,..), so after you import your assets you have to modify them to suit your project; Cinematic/PC assets need to be optimized for real-time or mobile and mobile/low-quality assets need to be modified for target quality.

The problems above can all be conquered and the assets can be very helpful and save you a lot of time, but as a beginner, it will only bring you problems, so focus on becoming a better Unreal developer and start asset bashing when you level up.

You should focus on understanding how the engine works and learning different development concepts. Courses from the unreal learning portal and official documentation are good for theory. Also, hands-on learning is important, so you feel familiar and comfortable with the engine.

Exceptions

There are some situations in which I think asset bashing is acceptable(when you are starting out).
If the assets you are downloading are fairly simple and you only want to use them to prototype and play around.

Asset packs like character animations, materials, meshes, and soundtracks are fine. They can be useful for exploration and maybe quick prototyping or scene dressing.
Again I wouldn’t try and do something crazy with them, but even if you are starting out, they could give you some insight on what the final assets should look like.