It always depends on how you use the meshes 
-> for things that are in the background (so far away from the player) + cant be reached by the player you can use meshes. You can also use a combination between meshes and landscapes. e.g a mountain that is a landscape + for the details it also has a mesh on top of it
e.g here in sykrim you can see this method: ://static-5.nexusmods/15/mods/110/images/2115-1-1322657788.jpg
white = landscape
grey = mesh