You can create fairly complex parts. Hell, I designed the 3D-printed glasses I wear and accessories in FreeCAD 🙂

What it's really not terribly good at is assemblies. If you need assemblies and simulating moving several parts together seamlessly, look elsewhere.

Another CAD solution you may want to look at is OpenSCAD: it's really nice if you like the programmer approach - i.e. you design the part by typing the parametric sequence to create it in a text file. It's great for parts that need to be configured with parameters. But it's all kinds of awful for anything curvy, and it doesn't do assemblies.