I believe your assumption is true as injection moulding is one solid part whereas 3D printing is done in layers meaning it may easily break along these layer lines. Also injection moulding is usually a solid mass, but printed parts usually have different infill and it is difficult to get 100% infill like injection as there may always be bubbles or groves forming at the layers. Unfortunately I don’t know of sources it tests to prove this.
This is a really difficult topic to accurately answer, partially due to the differences you encounter from one printer to the next, and also because “strength” is a very vague term when you’re talking about material properties.
From my own experience and others out there, it would appear a part printed at 100% infill on an appropriately calibrated printer can handle similar amounts of compression stress (squishing) as compared to a part made using conventional manufacturing methods and of the same material. The real issue happens when you have tensile stress (pulling apart vertically) and shear stress (pulling apart horizontally) on a 3D printed part. 3D printed parts fracture at much lower tensile stresses than injection molded parts, especially if the part is printed such that the z-direction will be bearing the brunt of the tensile force. Careful orientation of the part when printing can allow for the part to have very high tensile strength (comparable to injection molded), but there is very little that can be done to increase shear strength. Shear strength for a 3D printed part will always be lower than an injection molded part because you’re either pulling apart the from top to bottom or from side to side, and there’s no orientation that will correct that (with the exception of angled prints for SLA, but SLA resins tend to be very brittle and have little flexibility).
Sculpteo has some good information on material properties here 81.
