Quite often though, they aren't using the actual rendered mesh, rather a cheap approximation.The library is always expanding, with more trees & greenery joining the collection. I think the most common use case is in games that are based indoors, like your Quakes and similar, where the walls themselves use the mesh for collision detection. There are some cases where games use actual meshes for collision geometry. And you can get good results by tweaking the sizes of the primitives so there aren't too many edge cases. In most cases, the collision either happens or it doesn't, rarely is there the edge case where having your player notices that something should have collided but didn't(or the other way around). Games have a lot of this 'fakery' type stuff going on anyway. In my current project, pretty much everything is going to be based on sphere colliders. Most games simply use a few primitive colliders that can approximate the shape of the object. Another part is the simple amount of triangles involved. Part of that is the convex/concave issue(look it up). There are certain requirements for using meshes as colliders, and many games in fact avoid the issue completely. I can vouch that the issue has nothing to do with the modeller you are using, but rather what you are attempting to do with the model. I have searched YouTube, Reddit Forums, and Unity Forums. If there is any possible way to get Blender to work properly that would be great.

If I have to go with buying Lightwave It will slow me down dramatically. Does Unity no longer support Blender anymore? The only other modeling program I can purchase is the new Lightwave for $995.00 USD. I don't know if the model will actually fall through the terrain or not but when I fly my sphere into the model, it does have the collision. The model then has collision but gravity does not work. I have to click the Kinematic checkbox which turns off the gravity to the object. When I had a RigidBody to it, Unity throws an error saying that non-Kinematic mesh not supported since Unity 5. Then Imported the model in Unity 2018.3.0f2 and click on the model and clicked on Generate Collision. I have set the transforms to 0 on the X, Y, and Z. I have created a simple test model in Blender 2.79b and 2.8 Beta.

Blender 2.79 b