So, we got a bunch of “houses” that the player can enter into. They have irregular sack like shapes and we need to figure out how to do collision properly.
Is complex collision THAT taxing or would a bunch of UCX meshes slow down more?
What about tunels? It’s practically impossible to properly cover those with collision meshes.
Up you go!
“Complex as simple” collision will take the exact copy of your mesh as the collider -> so the mesh gets rendered twice = performance
So actually it depends on your mesh which collision type you should use. For a tunnel you could try to create a basic collision out of boxes -> depends on your game and where the player can go (e.g at the bottom parts you could add a detailed UCX collision and at the top just a very basic one) 
Look at this thread. https://forums.unrealengine/showthread.php?70023-Feeling-Silly-But-How-to-Import-a-Room&highlight=tunnel+collision
Use whats works best in any given situation. I was able to cover three buildings with just two UCX. I usually use CAS for ground as most times it’s just another primitive. Buildings I usually break it down into two parts as to whats a facade and what is deco as be it UCX or CAS the math is still equal.
The thing though is no one could come up with whats reasonable for any given situation so you really need to see the impact yourself via load testing what is assumed to be a limit or limitation. Since I usually start with the facade work first, more or less painted boxes, I’ll just take what ever I’m working on and dump the entire load in to UE4, give everything CAS, just to run around testing for scale and even with what would be consider less than ideal I still hit high frame rate numbers.
The way to think of it though is 1k = 1 buck and there is no point in spending 50,000 dollars on a five buck asset which is a fancy way of saying just do the math. 
Well what I’m mostly curious at this point is whether it’s viable to create a low poly version of the meh, set it to CAS and disable the renderer.
What you described is the process for making a UCX but just adding a unnecessary step.
Yeah but a UCX can’t be concave, can it? What I need IS concave collision, doorways, interiors etc.
Well I would have to see an example but over time UE4 has gotten better at figuring out what is convex and what is concave, what did not work before now works in more recent versions of Unreal 4, so the only way to know if it will work or not is to try it but as to theory if your LOD version is into the kilobyte territory I could see doing it this way just being another way of doing it. .
So as to the OP no it’s not that taxing as to performance but rather wasting resources best spent else where if excessive and of course the need as to best practice of keeping things neat and tidy.