Mycelium can develop itself not only as a growing material but also as a sustainable one:
it is renewable
it can grow on (and it "eat") food waste
it is completely biodegradable and it can become nice compost to enrich the soil
It is possible to have very different materials depending on which kind of substrate and strain is used, you can have something really strong (like wood) or something really fluffy.
If you want to obtain a hard material you should better use a "heavy" and compact substrate, like coffe waste for example. If you work under pressure (by keeping the substrate in the mould under pressure) you'll obtain an even more stong and compact material. The strain is also important, because not all the myceliums have the same features: Reishi is a beautifoul medicinal mushroom that can develop a very solid and woody mycelium, but recently I found out that also some kind the psilocybes cubensis (a magic mushrooms) can work really strong on the right substrate.
If you want to obtain a fluffy material, good for insulation for example, you should use a soft substrate like husks or straw, and a less woody mycelium, like the oyster one.