My personal feeling is I've used +/- modifiers for decades. I have used them in the case of cover. I am not annoyed/confused/cognitively disassociated from cover using modifiers vs adv/dis. So on that score, I'll keep them. Unifying mechanics for the sake of unifying mechanics is a bit silly, imo. Just be warned: using Adv/Dis gets rid of some of the granularity that +/- can bring. Effectively it gets rid of any minor penalties and major penalties (dis) can be negated by major bonuses (adv). Certainly simpler, faster, etc. But that is where the weird stuff comes in. If you're in the Gamist camp: Go for it. But if you're a simulationist at heart, use cover as is.
The cases where Adv cancels the Dis of Cover can occur more frequently than we might realize at first blush. Raging Barbarian, Sneak Attack, Faerie Fire, etc Using +/- ensures that cover is meaningful in all situations.
One way to think about it: Adv/Dis is a shorthand for active combat modifiers, ie stuff that people do (sneak, cast a spell, rage, etc). The +/- for cover is a passive combat modifier, ie stuff that just happens.
If a fight occurs on a swaying rope bridge and the DM wants to mimic the swaying in the wind hindering combat. He could use Adv/Dis, but again, it gets weird: My rage/sneak attack/Faerie Fire is canceled by the swaying? You could justify it by saying it is really windy and go for it, but I for one would prefer a -1 to -4 penalty depending on how much sway there was (perhaps a 1d4 at the beginning of each round representing the gusts).