You don’t understand - maybe you have never worked with it.
There is nothing predictible about a physics engine. Ever.
Its got literally nothing to do with being good (look at the invidia guys) or being bad (look at epic with Chaos). You inevitably will have issues due to the sheer nature of physics (what we understand of them, and the complexity of the calculations).
The only reason its Ok on cinematics is that you can do takes until you finally manage to get the effect you want. And then you save it and re-use it.
Similar to archviz. You wont be throwing projectiles at it, etc.
The moment you need it to function (say its a big rock in your landscape) then you cannot rely on complex as simple.
That’s all there is to it…