TressFX is free to use, source code is free to download, it is free to be implemented in any game, devs are free to change the code as they like when they implement it.
Gameworks is not free to use, the source code is basically hidden, you aren't free to modify or distribute modified/improved versions of it.
AMD gave devs a piece of software, an idea and a starting point to do with as they choose, Nvidia is trying to gain control, the latter is bad, the former is useful if devs want to use it.
The main issue with god rays is, and I've already found this in some inside locations, is that god rays are awfully done. One window had light leaking through and the entire scene basically blurs out. It's like walking into a bright room but your eyes not adjusting to the brightness. The Far Cry 4 picture looks frankly washed out and useless.
You can also see from the fallout 4 picture how this massive performance sapping god rays for ultra realism in the lighting... isn't even realistic. Look where the sun is compared to where a lot of those rays are falling. It's providing an irritating indoor overly bright washed out effect and outside it's very unrealistic in multiple areas. "We need huge power to give you realistic physics effects.... please ignore when they aren't realistic as we can't really explain that one, thanks, Nvidia".