Unity Occlusion Culling - Best practice ?

I saw the previous threads on this topic and I am still not sure how good occlusion culling works.

In my project, sometime it works perfectly. Other times it doesn't. Two major issues,

1. Hides objects which it shouldn't hide. For example, hides a wall which is visible to the camera not occluded by any other object
2. Doesn't occlude objects behind a wall, when the camera is actually inside a closed room.

Because of these issues I am thinking to implement some kind of manual culling to improve performance. I just want to confirm if I am not following basic practices. Is occlusion culling stable and works perfectly for anyone ?

 

Umbra is a hot mess sadly.