Ill explain the issue in more depth.
Firstly, the knockdown mechanic of the game is very powerful. It puts a player to the ground where they have increased spread, no movement and obviously are disorientated. This is usually ok, when knocked down from melee, as they are close, so the large spread and no movement means there is at least a little chance to kill a player that has hit you to the ground.
Grenades knock you down from range. You cannot return fire if they are any further than short range (effectively), making you essentially a dead man if you are on the ground while someones firing on you.
Again this would be ok if they had sacrificed the 3 seconds to cook their grenade, in which time they are unable to shoot and are vulnerable, and it requires at least practice to time a grenade to explode exactly on an enemy.
The problem with grenade shooting is that it is so easy to do. And so incredibly effective. The hitbox to explode a grenade is massive, much, much, much larger than the grenade itself, you only have to shoot in the direction of the grenade to blow it up.
So by merely throwing a grenade at an enemy, then shooting at the enemy, leads to not only your bullets hitting but also the grenade, essentially instakilling them.
It is so easy to do, you can easily perfectly explode grenades on targets, taking 80% of their health away and knocking them down within a second of seeing the enemy.
Again, this would be ok, if grenades were more limited, but they recharge. If everyone started with one grenade, and that was it, it would still be dangerous, but no where near as abused. Because they recharge, there is no reason not to grenade shoot in any fight, whenever you can.
In comp matches at the moment, and i use comp as the example as in comp matches the game is being played at the highest level, by players who play the most and know the game inside out. Grenade shooting is more than abused. Often, even at comp level, if you see an enemy and they see you, it is more effective to let them shoot at you with their huge spread and instead of returning fire, throw a grenade at them and shoot it. This is happening a huge amount. Whenever anyone enters a room, they grenade shoot the first person they see, 5 players doing this means most of the fights are decided on who grenade shot who first. The issue is that by the next spawn wave, everyones grenades have recharged and the process can be repeated.
On pubs its not as overused, but that is only because people are yet to discover it. The last couple of weeks pub players are picking up on it, and the same is happening as in comp but not quite at the level yet. Problem is that pubs arent resticted, so playing soldier on pubs is hilarious, with quick recharge increased radius, increased damage grenades, means you can easily take down spawnwaves as a soldier.
If there was a demo function i could show you overpowered they are. Im afraid it isnt a matter of opinion, if you dont think they are overpowered then you are just yet to see them used properly.