I mean it's got element of growth as in you earn experience throughout the game and level up. There is also a skill tree and loots. Personally I think any game that has these elements are considered RPGs to me.
I am not sure if you could really call it an RPG but it certainly can be said to have RPG elements which might we as close as you get. Sure you level up and get more abilities and what not but it does not really feel to me as though I am roleplaying a character but just controlling something.
Yeah, It's got quite a bit of customization and damage is done in specific HP amount's that vary so I'm pretty sure it qualifies as a FPS-RPG game.