You're right, I don't want to be too dismissive of people using purchased assets in all circumstances. I remember playing Inscryption and was quite surprised at how many pieces of the game were bought from the Unity storefront and similar places and
listed in the game's credits. As you said, how it fits together is the big thing, and Inscryption did that really well with the particular aesthetic(s) it had. I guess the bigger issue with stuff like Choo Choo Charles is that it doesn't have much of a substantial or unique visual style that makes it feel cohesive in the same way, so it just has that particular Unreal Engine plasticine-mushiness.