"Wood" is more generalized a term as "metal."

The "hardness" of the tree, the surface and chemical treatment of the wood, the moisture content and deterioration at the time we look at the support, the shape and design of the support...

All these factor in what could be determined the friction factor at a given support. To that end, I would never choose to go with wood, but if I must analyze it, I would argue that all wood supports are assumed to act uniformly, and be a range of 0.2 to 0.7, and note the likelihood they will not act uniformly and state a "normal" analysis budget does not permit looking at every combination of friction factors available.

But I'll also note the same situation exists with metallic supports. We don't know if they rust or bind, and we assume they do not and leave it to maintenance to ensure it.