The biggest problem IMO is that they never finish new features. They start the work on implementing new tech/assets/plugins, then abandon them halfway through just as they are becoming useful for prod. There are tons of would-be-amazing tools but they are all stuck at "0.x-preview" versions, and eventually after 5-10 years either stop working completely or are over-shadowed by newer, shinier assets, which often re-invent the wheel and/or do things worse than the previous attempt. I stopped trying out new tech until its 1.0 (or preferably later, 2.0+ is safer) because I'm afraid to be bitten (again) becoming dependent on abandoned plugins and have to at some point update to something else. It's a lose-lose-lose proposition: Unity throws away time and money re-inventing plugins, we throw away time and money having to port to functioning tools, customers lose time because they get to discover bugs caused by outdated plugins/assets that cause weird errors that are hard to track down.

I think what you're describing is a symptom of the issue. The issue is talent churn. Bright folks who start a feature get poached and leave and the feature dies on the vine. Or that the feature, which started off great in it's own little corner of the engine, was a mess to integrate with the rest of Unity due to it's architecture and the fear of breaking backward compatibility.

The problem now comes not from tech-previews but from the quarterly forced releases because it's now a subscription. The entire business model is flawed and outdated. Same with Unreal. The difference is Unreal tech is exponentially better than Unity architecturally and they know their audience very well so they were able to get in with virtual stage production, games, movies, you name it. They were successful in expanding beyond their core. Unity, can't. They don't know how.

It's a tough spot to be in. I knew my place when I shut mine down and open sourced it. I couldn't compete. For Unity, they have a loyal fanbase that wants them to succeed but I'm afraid it's going to take breaking everything they know in order to do it.