imho FAF is fine without massfabs, just don't overbuild power, or if you did massively overbuild - you can just reclaim it.
imho in BAR there are some things that prevent turtle gameplay, and t1 massfabs are not very efficient and require space and they explode...
BAR only has 1mex -> t2 mex -> t3 energy (afus) + massfabs
while in FAF you have t1 mex -> t2 mex -> t2 mex with storage -> t3 mex -> massfabs or sacus
you go from 2 mass per mex point to 27
while in BAR it is only like from 2 to 8
basically what i want to say is that FAF already has plenty of ecoing opportunities and i would not want it to have another somewhat efficient way of ecoing.
Also often FAF maps feel like they have more space, and building explosions feel smaller, while in BAR you have less space and building explosions feel bigger
Edit: You can also make an energy storage
i kinda like cheaper and bigger energy storage in BAR... but in FAF storage space is also tied to overcharge, so making storages cheaper buffs the overcharge, so you can't do that for no reason
Edit: also agree that massfabs in BAR are a big noob trap. Solar+massfab is like 22x less efficient than making mexes