Is combining materials the same as combining textures into an atlas?

Having many materials can be of detriment to your performance, however the main performance loss would come only if you are using GPU instancing. GPU instancing allows objects that use the same mesh and material to be drawn together, which can give a huge performance gain in scenes with many objects of similar appearance. The key thing to note is that even though the mesh and material need to be the same, you can still set different parameters (i.e. a different colour) per object using MaterialPropertyBlocks. One main caveat with this is that textures cannot be set in this way, so instead if you wanted a different texture on different objects you would need to use an atlas and pass in the relevant atlas offsets to use. Using texture atlases can also be useful for cutting down on memory usage, although it really depends on the scenario.