Heya, I have look extensively into multi-projector curved screen setups with edge blending software. I'll type some random thoughts as they come to me:

Firstly is the resolution of 120 Hz projectors which are 720P and quite low.
Stay away from software edge blending as all it does it cause problems.
Of course the size of the screen is relative to how close you sit to it. Bigger isn't always better, especially the larger the image the less powerful the light of the projector creating the larger image.
I've never seen any projected (front or rear) image that will have the clarity, brightness and motion qualities of a 120 Hz LCD. Just isn't going to happen.
Required to sit in a dim/dark room.
Required to have a large space behind the screens if rear projection, or a large space in front of the screens if front projection. With front projection you run into physical issues with the projectors. How close will they be to your head? 3+ projectors put off a LOT of heat and noise. For someone like me that is used to computing in silence, that is a huge issue.
Projectors usually have higher input lag and not favorable motion blur compared to fast 120Hz TN panels.
Re-occurring maintenance costs with bulb life. Not a trivial issue when you have three or more $300 bulbs to replace every ~1500 hours.
Brightness that dim's over time/use.

It's great if you want to play multi-player games with buddies in your room all the time, not really worth if for solo computer gaming.