Yes, that is also true, printed object are amazingly tough. First impression usually is that you can tear it apart with your hands, but a well printed part won't go down easily. However, in very many assemblies plastic parts will join against metal parts. If you don't factor the forces pushing against that plastic part and you size it as a metal part for example, you stand a good chance to get it delaminated. A good strategy is to design and print the part so that the force pushing against it goes the other way around, squeezing the layers together, instead of working against them. It obviously is not always possible, sometimes because of the shape of the part, sometimes because there are multiple vectors pushing against that part and you can only satisfy one or a couple of them.

Anyway, on topic, before adding 3D printer capabilities to a CNC, one should also check that the machine can sustain decent travel speeds, main reason being oozing during non-printing very slow moves.