You forgot a few minor details. Both mass fabs are more or less duty in combination with a mass fab manager (which should in included in faf). Since you sometimes need in late game for different upgrades and or using reclaim for spamming exp or big stuff like t3 arti lots of e you need spare energy. Means you need to built some energy reserve income anyways which can be used by having mass fabs. So far so good. T2 mass fabs have disadvantages as well and i will try to explain why i think the change of t3 mass fabs if needed at all is a mini change.
T2 mass fabs advantages:
-cheap mass income for low cost
-good energy buffering
T2 mass fab disadvantages:
-terrible pathfinding mess lots of buildings needed 1 building t3 mass fab vs 28 buildings t2 mass fab
-low life with chain reaction ability not so nice even weak bomber can easily kill farms
-built time of 28 t2 mass fabs to rech income of a t3 mass fab is 14,000 built time a t3 mass fab needs 5,000 built time
T3 mass fabs advantages:
-one building in between factories cannot chain react explosions
-good hp so it will not be killed by single t1 or t2 units
-very good mass addon for late game base to spam t3 units
T3 mass fab disadvantages
-lots of space needed for factory mass fab farms
-you need the factories prebuilt in a correct way
-you need lots of e income prebuilt and not good e buffer if you dont have a huge e storage since you can only switch 3500-e on and off
Watching all those i would advise to do only 1 change to them and they are great. Since they have this very big ammount of e usage they cannot be used good to buffer you energy.
-> every t3 mass fab gives 10,500 energy storage
Thats 3 seconds of energy usage. That would justify their insane death explosion much more and improve handling.
EDIT: Here to give an example i used all my efforts and played this game for you to show the use of a t3 mass fab for you played in a real 1200+ game 4v4, the game was turned was interesting