Injection molding can't work with many geometries; for example, complex enough boat has a lot of places where the material has to go along a non-convex paths. 3D printers can print anything, say DMLS, including closed volumes (how to remove material from those volumes is another question). Having many heads the process can be sped up - but with DMLS it's mostly moving mirrors which needs to be parallelized, as laser beams have little inertia.
Comparing with injection molding it seems IM is a good shortcut for many - not all - cases, while 3D printing is a fundamentally generic technology.
Comparing with injection molding it seems IM is a good shortcut for many - not all - cases, while 3D printing is a fundamentally generic technology.