The issue is that it's the same advantage as when there weren't any timers at all. Additionally, it is bringing back an advantage Riot intentionally did away with. Timing red and blue buff, as well as dragon and baron, was an advantage that players who remembered to do this could gain over players who did not do it. The issue is consistency, I think that removing the sound and visuals would be just as good a fix as adding the timer for it since there is info to give it away anyway. As it stands, it's an arbitrary advantage that seems out of place given that they removed the need to do this for buffs when you take/witness them being taken (which witnessing the flash on a lane opponent is the same as witnessing drag bc you just time it 2 seconds before).
The biggest issue is that by removing this from red and blue and any time you're the team that takes the objective, you create a situation where the game inherently favors players who have played longer. It took me a while bc I'm ADHD as all hell (clinical diagnosis and can't take stimulants so no meds for it, just have to work behavioral stuff on it), but eventually I got to where I timed stuff reflexively bc I did it so many times as a jungle main season 3. I still time drag every time it flashes, and prompt my team to set up when it's almost up, but it took me having to time a LOT of objectives before this became habit, and if you're new to the game the only one you have now is drag when you don't take it. That means someone learning the game now is going to take a LOT longer to make this a habit than it took me, and for the time that I still do it reflexively, I still have an advantage over that player, which doesn't make sense bc they removed it for other stuff.