It is functional, but turn off all of the assist and you'll notice how much you'll miss.

I play The Finals on controller, and maintained a top 0.1% position on the Power Shift leaderboard for 6 seasons straight with all the aim assist options completely disabled in mixed-input lobbies. I keep it turned off partially to prove a point - that raw controller input is competitively viable if you put in the work - and partially because the aim assist pulls toward the chest/center-mass and I like to aim for the head, so my aim is actually worse with aim assist turned on. I also manage to do this because The Finals has the best controller-tuning options I've ever seen in an FPS.

Game sense/awareness is much more important than your input method. Switching from controller to KbM doesn't make you a better player; it might make your aim better, but aim is only one small piece of what makes a player "good" at a game. Knowing the layout of the maps and what routes you can exploit, understanding the game mechanics and game mode objective, predicting player behaviors, communication, mind-games, adaptability... any one of these can make up for poor aim. This applies to any game.

But even if aim is the only thing you're concerned about, you can still aim-train on controller. It's different, sure. But practice is still practice; if you put in the work, you will get results.

The best input method is always going to be the one you're most comfortable with.