In unreal and unity, most of the third party code I’ve tried to use ended up unusable because of either bugs, performance issues, or limitations. There are great exceptions but it’s been frustrating overall. The ones that do work, well, the documentation is often terrible and it takes longer to learn and integrate than it would to make your own. Finally, unity has a history of retroactively changing their terms of service to squeeze more money out of you for asset store buys, so it’s scary to trust them. I write a lot more of my own systems now.
unity (and maybe unreal) have a real problem in that the majority of users are hobbyists, students, and new programmers. None of them are really willing to pay much for 3rd party code and they need $$$$$$$$$$$$$ of support because they are new.
For example make a JSON serialization library and you'll be asked "how do I spawn enemies from JSON" which is arguably entirely orthogonal to your library but don't answer and get downvoted for bad support. Do answer and you'll just be asked more questions about how to adapt your example to their personal project and you'll have to teach them programming in the process.
These incentives make it almost impossible to make money selling 3rd party code (plugins/add-ons). If you charge what it should really cost given the amount of work put in the market won't bare it. If you charge what the market will bare you'll go broke.
Maybe Unity should split the market into "Pro" and "Hobbyist" and the Pro market would have prices more inline with what it actually costs. Check out the prices of libraries like Radgametools.com (you'll have to google the prices) for comparison.
Unity also changes their APIs around with every tiny version increment, meaning that any app store package has to be constantly on top of installing new versions of unity and updating all the little parts that break every week.
