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The issue is not so much that it's cellulosic as that it's lignocellulosic; we have a pretty good grasp these days on cellulose (and hemicellulose, but we can handle that too so I'll skip over it). Lignin is (oversimplifying, but not by too much) what makes the difference between soft paper pulp and hard woodchips. The lignin crosslinks the (hemi)cellulose and makes it much harder to access for the enzymes we'd use to break down the cellulose, while itself being very difficult to break down (it's thought that the whole reason we have coal is due to how long it took for fungi to evolve the enzymes needed to break down lignin effectively in order to degrade wood).

You need a bunch of equipment and/or biochemical processing to break down lignocellulosic plant matter into something that can be efficiently fermented (keyword: "lignocellulosic biomass pretreatment"), so while thay may be available at a lab scale, it's not necessarily possible on an industrial scale (yet).

As one example, there's a method called "steam explosion" where you apply very hot (around 160-260 °C) and high-pressure (tens of atmospheres) steam to the biomass, then release the pressure relatively fast. The hot steam causes chemical reactions like hydrolysis, and the pressure release breaks down the material physically as the steam expands. Imagine the sort of equipment you'd need to do that on an industrial scale. Nowhere out of reach for modern chemical engineering, but someone has to build it.

Now, as you might imagine from how relatively non-woody switchgrass is, it doesn't have a ton of lignin compared to trying to ferment woodchips, but it still has enough to be problematic, and in fact there's research being done on how to reduce its lignin content, such as by genetic engineering.