Trees and terrain are now supported so that should not be the problem.
I would suggest when loading your project to monitor GPU VRAM and system RAM usage. VRAM you can check with GPU-Z
, system RAM with Task Manager.
To save resources, hide anything that is not in view, reflected, or casting a shadow in the iClone Scene Manager.
In iClone, there is now a Minimal Shader settings that will further reduce the resources used by iClone.