Have you seen the old Slann? It's kinda infamous for being next to impossible to assemble.
Anyway, I haven't seen the Terradons in their separate parts, but from the pictures of the assembled models it looks to me like there is plenty of room for pinning.
And if there isn't the design of the model is to blame far more than the material.
Take a look, for example, at what kind of pose Freebooter is able to produce in metal while designing the parts in a way that makes assembly easy.