I'm not saying I don't want more strict 5e rules. Im just saying I don't mind variants that make sense.
Ever fire a bow? Ever fire one in a mock combat? It is considerably easier to hit a target from above than on the same level. Why? You can see the target better, and you aren't as worried about getting hit back especially if they have no ranged weapon. So you aren't as frazzled and you can see them better. And height does give you increased range. And it is harder to hit someone who is on a higher level than you. So all that makes sense to me.
Arrows never fly straight at a target if you have any range. They arc. It is hard enough to gauge the right arc you need when shooting 20 ft away. Throw you into the midst of a battle with targets moving, trying to avoid hitting friends, etc. It becomes way harder. Imagine firing between trees to hit a particular tree only the trees all move. Give someone height and it makes a ton of difference. You can see who is who easier, see more of your targets, etc.
And as for as flanking goes, I do think advantage is appropriate. Go play nerf swords with a few people. Have two people attack you at once. It is SO much easier to land a hit against someone when 2 people are attacking that person. SO much easier. It is NOT easy to get behind them when soloing them.
