The only time BSP's gave me trouble was doing a stress test with a crazy amount of square blocks.....all with lightmaps ON, which gave me Out of Memory errors and an aborted build.

Once I turned lightmaps off, the level compiled fine and ran fine, which then you just have to deal with just monitoring how many triangles the BSP blocks add just like models.

I prefer level blocks when it comes to quickly creating large blocks or areas that need texture repetition, like a big building, a big wall, or a large floor. I dont want to have to unwrap a 3d model every time I want to create a new large surface. Also, blocks do not add to the entity count.