Didn't see this covered and I'm not sure if this was already changed, but:
On the contrary: If the seed-drop rate from a single tree is less than 1 seed / tree, then it is not "hard to propagate", but impossible to propagate and the trees will go extinct! Harvesting 100 trees and getting 80 seeds means the next round you have 80 trees and get only 64 seeds, then 51, then 41, and so on, until there are no trees left.
This might be fine for a single player world where you can go explore new areas all the time, but even here this is mightly tiresome if you just want to do a nice build with wood X and have to do longer and longer travels to get some. In MP, it will be a desaster, as busy players will deforest all the nearest areas and leave no trees and there is no real way to grow them back, as every tree results in less seeds to replant.
And the whole point of new wood colors is so that people can actually use them in their builds. That's the whole point of adding new stuff, isn't it?