I dabbled with 3d printing and found that the airframes were too brittle to handle crashes/hard landings. Unless you are an excellent flier who rarely/never crashes, it's not worth it in my opinion. Where 3d printing shines for this hobby is for making simple stuff like control horns or a custom motor mount.
Injection molding doesn't have layer lines and would be in general more durable than a 3d printed part (especially if comparing the same type of plastic for both processes). I don't have any full sized injected molded planes, but many of the props I use, and micro quad airframes are injection molded and do well. In general, plastic doesn't scale to full sized planes well - it simply is too heavy when thick enough for durability, and then if thinner it's too weak to handle the rough treatment I put my planes though.
I'm with @joelspangler on this one. As precise as injection molding is its greatly limited by the shape you want the part to be. With a 3D printer you can create almost any shape you want and easily switch to printing a different part. I do think injection molding is useful when producing high quantities of parts