This is my conclusion. I spent 2 entire days playing a controller and 2 days playing with mouse and keyboard. No, you cannot spot the difference at all WHEN playing.
NOW HOLD ON, I am not done yet.
Mouse and keyboard can give you a HUGE advantage because PC players can flickshot and turn around swiftly. I am probably one of the lucky few who does not feel any input delays with my mouse, so my experience felt flawless. Movement feels right, despite we cannot crouch while walking, it's just part of the game. It's the same thing with controller. Those who played this game since since 2010, including me, will easily get re-used to it.
I felt sluggish when using a controller, but I was still able to play competitively, thanks to my muscle memory. There were so many things I wish I could do like looking around 180 degrees, do insane flicks, etc. I even thought about the many insane things mouse and keyboards players could do when flickshoting like 4 flawless rapid ridiculous flick-headshots with the DMR in swat (I think it's easily possible). You can still flick with controller but it's "limited". But here's the thing, once a controller player starts to flick comfortably, I think this is terminating the fairness between both inputs.
When I switched to mouse and keyboard, I felt there was a hit detection problem all the time when playing with Mouse and keyboard. I felt something was wrong but I also felt maybe it's me. I still felt my sensitivity was on point and my mouse was flawless, so I couldn't really tell the difference when I switched. I have to say if you are not focusing, tracking your enemies, lining your cross-hair on the right position, you chances of losing your fights increases dramatically. When using a controller, you don't really have to focus as much as a MKB player, thanks to the assist. But, I was more comfortable using mouse and keyboard, however I still thought the game had hit detection issues.
So.. since I recorded all my games, I had to slow mo my game footages. AANNNND I think I can finally say..... that YES.... Aim assist is TOO advantageous.
You can finally spot the difference. When I kept missing my last shot with the DMR as a MKB player, it was NOT hit detection the issue, it was because I was too used to aim assist. When using MKB, my shots always, ALWAYS landed literally AN INCH away from the enemy's head and I think this is a problem because it happens A LOT less when using a controller. The slow-down is creating discrepancy. MKB users have to stay focused to perhaps beat controller users, but it shouldn't be that way. Controller users have the edge while having less focus on the aim.
Some will say "YEAH OK, you just got to practice" (I admit it, I said that some times), some will say to reduce AA, some will say to reomve it.
But some will say that INPUT-BASED matchmaking should be implemented, so cross-play can be implemented, and I think that's just the best solution.