A landscape system is obviously superior for an open world, but if your maps are small like Counter Strike or TF2, then static mesh will work absolutely fine, and you’ll have greater control over optimisation.
Terrain that you will never reach, like skybox terrain, is best done as a low poly mesh with baking from high poly. If I use a tool like pro-optimise in 3D Studio , I can take a 100k volcano, and bring it down to below 10k, and it will appear to look exactly the same because the light-mapping and normals will be baked into a texture using the 100k version. No one will ever get close enough to know the difference because its outside the playing area. You can also make the detail stream-able, like with a landscape system, by using tessellation.
You can use LODs with static meshes, and even tessellation instead of LOD, so there are ways to optimise static mesh terrain when you do use it. You can use the vertex colours in your modelling software to paint where different texture blends will be. You can leave it simple and just do black and white, one channel, and divide the mesh up per material (grass/dirt, grass/sand, dirt/rock, etc). Or if you don’t want to divide up that much, you can use all 4 channels RGBA. It’s up to you 