advantage / disadvantage mechanic is 2 rolls to succeed or 2 rolls to fail. Disadvantage which the mechanics do match the actual table top mechanics in that sense basically say if you have advantage which you would when flanking or from height means you have 2 chances keeping the better roll on a 20 sided dice to hit. If that roll is 20 means your chance is like 10% instead of 5% vs if you were at disadvantage you would have basically a 2.5% chance to hit because you would need to basically roll 20 twice back to back disadvantage means you take the worst roll of the 2 rolls. Im just using the example as it explains the mechanic easier. Essentially Advantage accounts the enemies armor vs your change to hit with the better of 2 rolls on a 20 sided dice vs disadvantage meaning your taking the lower of 2 rolls. So if your attacking from an elevated position then essentially you have advantage because your shooting from above meaning cover is more difficult and if you were attacking downwards defending is more difficult as you have more leverage in your attacks. The mechanics are based on the concept that it would provide an advantage like wise flanking is similar if targets are attacking your from both sides those targets have advantage to hit and vice versa meaning there odds are twice as good doesnt mean it will succeed but it massively increases the odds when you consider the average armor class of characters in your group being around 15.
If the enemy has advantage that 15 is turned into a 50/50 hit chance roughly same time disadvantage would if it was used against the enemy would put them at around 12.5% hit rate not accounting for the difference proficency bonus ect makes in those odds. With you always having a 5% chance to auto succeed and a 5% chance to auto fail. Based on what your fighting. Modifying the mechanic to be +1 or +2 just completely defeats the purpose of the games existing rule set. As do several other mechanics which im sure they intend to fix in a way such as wizards casting cleric spells and weapon swapping as they also break the games mechanics currently. In order to bring the rule set more in line with existing rule set. I can see some implimentation changes which make sense like the ranger had a large number of faults to begin with which still need alot of tweaking such as beast master hunters being way under powered in late game.
Same time I am sure there are tweaks that need to be accounted for in advantage / disadvantage as you would not have advantage at close range firing a bow from above the target or within 20 ft or roughly 6 m of the target. You would be at disadvantage countering the advantage from height. However from am melee perspective being 1ft above the target attacking down on the target would give you an advantage on attacks against that target as you would have more power in your swings swinging down on the target. Same time being 10ft above a medium size target you would not get that advantage because the target is out of range of your reach and vice versa. You get line of sight issues hitting a target at close range when directly above a target as you cant shoot strait down at it due to angle. This can even impact some abilities like magic missle which is basically an auto hit. Cover plays a factor there in regards to line of sight. There probably is some adjusting needs to be done with height / distance to determine advantage / disadvantage rates but essentially a target inside 6m would automatically apply disadvantage which im sure ive notice that on attacks prior with hit % decreasing till i got outside that range however the mix match of the 2 may be off. Only larion could really answer that question if its functional accounting advantage against disadvantage.
Long and short of it the mechanic should work as its intended in the games actually table top design. So I agree with advantage from flanking and height. As long as similar mechanics are applied with disadvantage which currently seems accurate.