Thanks everyone for bringing up spells. It's definately another source I hadn't thought of (beyond gm jusgment of course).
I love the adv/disad mechanic in 5e, but the implementation could be improved i feel. It's tied to some spells, some class abilities, the gm can assign it in aplropriate situations etc. I'd prefer if they were seperated out somehow, but that's a whole different conversation.
If i were to use something like this option, i might instead use a "fighting defensively" option. Under that option they forgo the proficiency bonus for all their attacks that round to gain half their proficiency bonus to their ac. That's not too math involved and doesn't then interact with advantage and disadvantage.