Yeah its hard to do a 1:1 comparison I bet and I am willing to give CryEngine the benefit of the doubt since it does seem more optimized for purpose, UE4 has gotten significantly better already with early versions not even supporting foliage options much at all.
I have no doubts that speedtree comes with its own set of issues and one I typically see in games is really horrible attempts at wind in both shaders and tree skeletons (if thats the correct term), I would actually go with the option of no wind until you are absolutely sure you even require it and have room for the overhead that comes with, its one of those eye candy features that doesnt typically impact gameplay in the slightest.
Its really all about trade offs thats what I like to say, its rare for a game to have the ability to crank every part of the engine to 11! An example might be using SubSurface Scattering with Translucency to get really nice high detailed individually modeled leaves which correctly filter the light and show the veins but having a scene with a big forest and trying to have all the leaves look that good will most certainly kill your framerate (be lucky if it doesnt crash tbh). If you batch them add some nice lodding textures which only do the SSS up close then you immediately see benefits and thats the type of flexibility in development UE4 provides, unfortunately it has a similar issue to CPP in that it allows you to write amazing stuff but it also allows you to make huge mistakes 