Designing for printing is a lot easier than designing for mass production. You can print things that would be difficult to mass produce by injection moulding.
The trick is being able to design something that doesn't have any open holes in it. Should be a closed solid object. Once it's got a hole in it, it's now no longer solid but conceptually rather like an origami paper-thin like shape with no internals. Solid-modelling software is generally easier but more complex surface modelling software you might run in to issues with this.
However, some of the printer driver software has gotten a lot better at handling these errors lately, such as GrabCad Print, so things are certainly becoming easier.