Does anyone have an idea why my trees (Speed Tree 8) look great in Speed Tree, but look like crap in engine?
Take a look at the screenshot. The Unity version of the tree is washed out, too dark, and just generally not receiving any ambient light.
Using URP, all lights are realtime with a single probe. (its a very tiny scene).
I think speed tree shaders have been broken in URP since the beginning as there have been several issues reported on the forums with no official fixes yet.
https://issuetracker.unity3d.com/issues/lwrp-shadows-applied-to-speed-trees-are-too-dark-and-change-their-appearance-depending-on-the-distance
The link above will give you some additional info in the comments and a link to a forum post that has a speed tree shader with a potential fix for your problem.