It is the hardest role cause it goes against FPS fundamentals by using moba powers.
So for example as a ranged hero you keep rotating back or to other side or forward depending on the push made by enemy tank.
This is how most heroes interact.
The thing with flankers though is they have the mobility to close distance fast and cut the rotation and get a elimination.
so your always going to be working against many players, unlike ranged hero duels were your fights are more organised and its mostly about who shoots who as you both have established positions.
But a flanker has to cut the rotation then establish a position or secure kill but atleast in teams with awareness you will be working against a team as no one wants rotations cut.
So once you reach ranks were people are aware isolating them to cut rotations will get tougher, but you will also get teammates who can enable you to do that flanking like another dive hero or a zarya bubble or zen orb etc.
All that said the flankers in overwatch have some of the best abilities in any game ever to aid in flank.
I mean tracer can literally blink 3 times and has a reset, sombra is practically invisible.
Echo can fly anywhere.
Genji has wall climb dash and double jump to take positions.
So the flanking part itself is not actually tough even though it should be apart from maybe genji and reaper.
The real issue is stealthiness of the flank to not raise awareness which is practically impossible most of the time in ranks with awarenesss.
So it boils down to can you steal that kill during that engage.
- Best case you engage get a kill and survive.
2.Worst case you die without a kill
3…bad case you kill but trade.
4.Okish you bait enough abilities and escape out for a later engage.
This provides opening for your team to push as they should, this again only works in team with awareness and in higher ranks.
So the reality is flankers are not as oppressive as nonmobile heroes think, the actual issue is they see them easily cutting the rotation with abilities, but the actual duel once thats done is a lot more balanced.
At the same time, while flanking is harder than other nice roles, it doesnt mean they are rewarded properly for it, its a high risk reward role,
Otherwise why would you play a blinking 150 hp tracer over a thicc Cree.
I really wish people can have indepth discsussions on these topics rather than flinging bags at each other based on their hero or role bias.