It depends on the game, and how aim assist and movement are done.

If a game rewards slow movement, controller tends to be better. For example, in Splinter Cell, your mouse wheel could adjust the speed you move at, and sometimes you really had to slow down to be not heard or seen. Controller does this easier and friendlier, from the angle of the stick. Granted, most FPS games don't really have mechanics deep enough where your movement speed matters or adds up, but it can. Realistic mil-sims for example add a massive muzzle sway for a while after you stop running. So jog-walk-jog-walk was easier, and controller made it simpler.

Aim assist is anther big one. And will get bigger as you get older. If you're still a kid in your brand new body and everything works, treat. Come back to me when you're on the wrong side of 60, with bad eyesight and arthritis. Suddenly mouse ain't so hot no more.

I'll give you a non-FPS example, but Diablo 4 beta was last weekend. I've been playing Diablo since '96 original. Always with keyboard and mouse, the way god intended. In fact, none of the previous Diablo games even had controller support (on PC) out of the box, only D4 does.

So I started with KB+M, of course. And the thing is, I haven't played a Diablo game since '12. And my eyes are not what they used to be. When I click on an enemy that is too far, my character moves towards them by itself, and that messes up my vision, because the movement is unexpected I momentarily lose track of both my character and the enemy. If the enemy is the boss and does a telegraphed swing I'm meant to see in that moment, I'm gonna miss it. I also frequently flat-out lose track of the mouse cursor when there's a ton of shit happening on the screen, explosions, flashes, mobs running around, I have trouble finding the cursor. So I was kinda having a bad time, playable but not great.

So on a whim I plugged in a controller, and BOOM, gamechanger. The biggest thing was, no more unexpected movement. Now my stick is WASD, and I know precisely where my character will go, and at what speed, from walking to sprinting, based on how hard I lean on the stick. Another big advantage was the rumble of the controller. Whenever I stand in fire, controller rumbles violently, so I know I gotta move. I no longer need to see it, or to see my health bar dropping, to realize I need to move. And finally, there's lock-on targeting now, with a right stick press, and easy target cycling. Meaning if I have a priority target, I can lock it, and that's it, I pretty much don't miss after that (soft of like aim assist in FPS). Took the game from manageable to downright pleasant and very accessible.

In short - it depends on a game and your physical state. I've been keyboard gamer the whole time, and started using mice when they started to become popular and games started to support them, in late '80s and early '90s. Hardly ever used controllers. But lately I got a newfound appreciation for controllers, especially in VR. VR controllers are basically regular controller sawed in half, and it also tracks its position and movement in space. And seeing as VR is very likely the future of gaming (if you played Half Life Alyx, you'd see why), controllers are where it's going to be at. You no longer aim with a mouse OR controller, with a motion controller in VR you aim with your actual hands. You turn by actually turning. Your body is the controller. So the whole KB+Mouse vs Controller is kind of becoming a moot point. Once VR hits mainstream, it'll be motion controllers > everything.