Hi everybody !
Currently the stupidest thing ever is updating our project to Unity 6.
We have a very large Racing game project, and have a lot of assets, and everything was fine when using Unity 2023.x.x, but everything broke down when updating to Unity 6. The game’s physics is broken and there’s no way to fix it. It still happens when using the latest version of Unity 6.
We don’t know what the heck has changed in Unity 6.
We have been developing for a while on Unity 6, and there have been a lot of changes from the source code to project resources, and it is not possible to revert it to use Unity 2023.x.x. Although we are using git for management.
I advise everyone that, if your project is big, it’s better not to update or use Unity 6. It’s too dangerous. Now we have to delay the release of the project because of this stupidity, and we don’t know when we will be able to release it.
It’s unfortunate but at least you can go back to the latest version of your project you made using 2023.x.x. You’ll only lost time. 
Well, that’s the only way now.
To go back to the commit where we used Unity 2023, there were a lot of changes. And we had to redo and retest a lot. It takes a lot of resources and money to do that.
There are a lot of level scenes designed since switching to Unity 6. So we will have to redo everything if we want to return to using Unity 2023.
What a stupid move to update to Unity 6
It’s really unfortunate that Unity 6 doesn’t seem to be clearly advertised as “preview-don’t use for production”. It’s not even “Tech” (which still it would be equivalent to “beta”). I’ve got a report from an user that Unity 6 was somehow auto-downloaded by the Hub, and asked me why my vehicle physics package is not working.
Physics-wise, there are non backwards-compatible API changes, as well as show-stopper bugs still unresolved, such as this one: Wheel Colliders drop through the ground if any settings on the Rigidbody are changed in Unity 6
Unity-6 is a preview version. Please, make sure you report any issues you might be encountering so that the Physics team can fix them!
Thanks!
Our car moves at speeds from 200kmh to 360km, and when it collides with a wall, it always inexplicably goes through the wall. We also tried it like setting vehicles rigidbody such as: Interpolation, Collision Detection, but it also doesn’t work.
Even when running at high speed on the road, the car is thrown up inexplicably.
These phenomena do not occur with older versions.
I would not use any non LTS Unity version for development.
Using Unity 2023 or Unity 6 is a massive risk, so best practice is use Unity 2022.3 until few months or years of LTS release of Unity 6 later in 2024.
Even with LTS would still wait at least few months, as the LTS on its release is the first true Beta in my mind, then start the final fixes.