Just sounds like a typical homebrew game. You need a lot of discipline and design fundamentals to make anything that scales at all well. Even a lot of commercial stuff, if you get a deep dive look at the internals they talk about what a mess they are.
Things like Unity and Unreal allow companies to compartmentalize they layers of the engine, effectively passing off to someone else to handle the nitty-gritty. This doesn't make them better, they're usually just as much of a mess or more, above the level of the stuff the engine gives you.
A lot really comes down to design constraints. It's far easier and quicker to mock up something that just looks great in 3D tools then shove that into the game, and fake as much of the "systems" stuff as you can, holding everything together with craft glue and sticky tape, rather than building a deep engine, then implementing the same stuff in a flexible way. Basically, for virtually everything in your game, faking it with mock-ups that do just enough stuff and no more is always easier than building a detailed system which can do the stuff you need, plus more. So there's always that incentive to cut corners in order to get good looking demo stuff out there that's going to push the project forward.
Code that sucks and bad design that means new features are half-assedly bolted on top of other stuff? Standard operating procedure in the game development world. It really is mickey mouse compared to high end commercial software. Like, nobody would tolerate the level of bugginess if it was the latest video editing suite or something, because those people need it to work because money is on the line. Shit is routinely shipped entirely broken in AAA games, which tells you the state of the industry and how much they've actually mastered the art of putting games together. Nowhere close.