“If VPD is too large, the plant stomata will close in an attempt to limit transpiration, which can result in issues like tip burn and leaf curl.” Decreased transpiration will limit water uptake from the grow medium.
I can agree that people can get too caught up in charts, but at the same time those charts can help a lot. A plant in veg can still grow at 30%, but it isn’t going to grow as well as a plant at ideal humidity conditions. Mold and bud rot won’t be an issue until his plants start flowering at which point 30% would be good