You have to factor in roll-off time for factories. They spend about 13 seconds building a t1 tank and 2 seconds for it to exit. If you have 8 engineers assisting, it would take closer to 4 seconds to build, but still 2 seconds to roll off. The difference between build time and roll-off time means there are diminishing returns for adding engineers. If you have enough hives, the roll-off time will be bigger than the time it takes to make the unit.
Also, more engineers = more juicy a target for t1 bombers or other kinds of raids. So there is increased risk compared to having multiple factories. And engineers take time to build. If you want to increase your production as fast as possible, it's better to have engineers make factories, rather than using the engineers to assist factories.
For structures: you can run into problems similar to the "roll off time" issue for factories, when you are making multiple structures, because engineers have to move around and there can be traffic jam. If you have 10 engineers building 1 line of t1 pgens, probably half their time is spent moving rather than building.
From an efficiency perspective, you also have to look at the fact that a unit or building that is half-done can't do anything for you. If you have a T2 HQ and two T2 support factories, and each one has 1/3 of a Pillar, you have zero tanks you can use. If you have only a T2 HQ, and it's assisted by 15 engineers, in the same amount of time you could make a pillar. This is more of an issue with big expensive things like T2 navy.