I'm not sure where this GPL confusion came from. GPL was designed to infiltrate the code base of the program itself, i.e. all stuff creating the final binary and the libraries it calls. It doesn't apply to external data, or external programs. External - meaning there is no function call. You can run GPL program and read its output from closed source program. And the opposite - GPL program can read proprietary data. There is one exception though. If the data is a script, and the script calls GPL code, then the script "links" with GPL, and therefore must be GPL. But art assets just can't "link" with source code from the perspective of GPL.
However, most, if not all believe that GPL will "leak" into the game code itself, since they've heart that this is what GPL does... This is why one should definitely avoid it if the whole project would not be released under GPL. Even with permission of the author, what about gamers that look into this and "feel" it's unfair. Making something is not just about the law, but also how people perceive your work. Especially the GPL guys are very "touchy"...
But IMO, GPL is a terrible license for the kind of people that will search for free assets. These are generally indies, one or few people that will have difficult time "protecting" a fully GPL game. Why "protecting"?... The biggest strength and biggest weakness of GPL is the ability of forking. Forking allows the project to live on, no matter what happens with the original author, but also means that anyone can make a spin off of your work, making it near impossible to have something like an IP. This is generally fine for most software (there you can patent), but when you are creating a game, you have particular vision. Imagine someone else taking the source, and doing all sort of stuff with your game/vision. Many would not like it. It doesn't mean that the author doesn't like to open source the game, or want to sell it. It's about GPL particularly, which was not designed to work for art / subjective topics.
I can give the following brutal example. Imagine you create some priest/nun assets and someone else use them to create spicy xxx content. You can't avoid similar things from happening with GPL, nor CC-BY-SA. Open source generally works best for bigger companies that can support their mainline code. For example Blender. There were a couple of Blender forks, but failed since the Blender Foundation has bigger community and more resources. Especially these days... But for indie devs, GPL means fragmentation of the community, and potentially - loss against another fork. Same with CC-BY-SA.
IMO, this is why Unity/UE4 became so popular with indies. Actually - these are easier as platform. We get basically free engine, doesn't need to release any code. There are many free assets, or cheap ones to buy. For $100 buck you can do simple games, and do not have to deal with all the drama. At least for games, this seems to work best.
What if you are not even an indie, but just a hobbyist? This is probably the toughest situation. Dwarf Fortress is one great examples - free, but closed source. If it was released open source, we would have at least 100 clones by now that very little gamers would be able to differentiate...
For modern commercial FPS, I don't see a reason not to go with UE4... It's built for FPS, you get the source, and it's practically free (good luck making 1M a year).
The problem is how UE4 works. It bakes the assets creating a "derivative work", then stores them into a package. And since this packaging and the new asset format is proprietary, you can't do it for CC-SA. However, IMO, it's perfectly fine to load these assets (from the original format, or other common one) at runtime. But again, while possible, this is not how UE was designed to work, nor Unity...
And the second issue is of course the license mess.
I'm not sure where this GPL confusion came from. GPL was designed to infiltrate the code base of the program itself, i.e. all stuff creating the final binary and the libraries it calls. It doesn't apply to external data, or external programs. External - meaning there is no function call. You can run GPL program and read its output from closed source program. And the opposite - GPL program can read proprietary data. There is one exception though. If the data is a script, and the script calls GPL code, then the script "links" with GPL, and therefore must be GPL. But art assets just can't "link" with source code from the perspective of GPL.
However, most, if not all believe that GPL will "leak" into the game code itself, since they've heart that this is what GPL does... This is why one should definitely avoid it if the whole project would not be released under GPL. Even with permission of the author, what about gamers that look into this and "feel" it's unfair. Making something is not just about the law, but also how people perceive your work. Especially the GPL guys are very "touchy"...
But IMO, GPL is a terrible license for the kind of people that will search for free assets. These are generally indies, one or few people that will have difficult time "protecting" a fully GPL game. Why "protecting"?... The biggest strength and biggest weakness of GPL is the ability of forking. Forking allows the project to live on, no matter what happens with the original author, but also means that anyone can make a spin off of your work, making it near impossible to have something like an IP. This is generally fine for most software (there you can patent), but when you are creating a game, you have particular vision. Imagine someone else taking the source, and doing all sort of stuff with your game/vision. Many would not like it. It doesn't mean that the author doesn't like to open source the game, or want to sell it. It's about GPL particularly, which was not designed to work for art / subjective topics.
I can give the following brutal example. Imagine you create some priest/nun assets and someone else use them to create spicy xxx content. You can't avoid similar things from happening with GPL, nor CC-BY-SA. Open source generally works best for bigger companies that can support their mainline code. For example Blender. There were a couple of Blender forks, but failed since the Blender Foundation has bigger community and more resources. Especially these days... But for indie devs, GPL means fragmentation of the community, and potentially - loss against another fork. Same with CC-BY-SA.
IMO, this is why Unity/UE4 became so popular with indies. Actually - these are easier as platform. We get basically free engine, doesn't need to release any code. There are many free assets, or cheap ones to buy. For $100 buck you can do simple games, and do not have to deal with all the drama. At least for games, this seems to work best.
What if you are not even an indie, but just a hobbyist? This is probably the toughest situation. Dwarf Fortress is one great examples - free, but closed source. If it was released open source, we would have at least 100 clones by now that very little gamers would be able to differentiate...
For modern commercial FPS, I don't see a reason not to go with UE4... It's built for FPS, you get the source, and it's practically free (good luck making 1M a year).
The problem is how UE4 works. It bakes the assets creating a "derivative work", then stores them into a package. And since this packaging and the new asset format is proprietary, you can't do it for CC-SA. However, IMO, it's perfectly fine to load these assets (from the original format, or other common one) at runtime. But again, while possible, this is not how UE was designed to work, nor Unity...
And the second issue is of course the license mess.