The title says it all
The FAQ makes it sound like you can use their assets, but they don’t recommend it.
But how would the player now I used it if it’s a fps premade?
https://support.unity3d.com/hc/en-us/articles/211950043-Can-I-use-Unity-assets-in-my-game-
They probably won’t, but I still wouldn’t use it for a commercial game. Write your own or use - for example - UltimateFPS, if you have the budget.
Okay thanks for the answers!