The problem really don’t lies in there. But sure some changes are needed on the way the heroes can use those abilities(mercy amp could provide less than zen amplification). Because of this:
Affecting the target vs affecting the source. Have 2 main differences
1- target amplified means everybody including herself deals more damage on that target. But barriers and any source of damage mitigation negate that benefit all times, also debuff removers.
2- damage on source, means that source it’s target agnostic, so can deal increased damage in any target, barriers included, only removed if the source of that damage amplification dies or stop to provide.
The problem with overwatch, without barriers or lesser barriers it’s a balance with 4 pilar things.
1- Damage, the way that damage is made
2- Fire rate, ratio per sec
3- Ammo, the downtime of effectivelly doing damage
4- Abilities that complement that
On damage side, if you got too many damage in a single shot. Means that to balance you need slower firerates or less ammo or both.
If you got a lot of “mini” shots on that, means that you need balance the spread, fall-off, their individual damage or their numbers (shotguns mostly)
High damage in a single pellet = less ammo or/and less fire rate
High damage in multiple pellets = more spread, less damage per pellet, less ammo or/and less fire rate
Low damage in a single pellet = more ammo or/and more fire rate
Low damage in a multiple pellets = more spread, less damage per pellet, more ammo or/and more fire rate
It’s just math and mechanics. Some heroes have more burst shots than others even dealing the almost same damage per sec. The problem it’s how that damage it’s made, if you find the root of the problem you solve the issue. About DB the problem lays the way they can be done and what kind of downside they represents. From mercy we got 2 downsides(can’t attack, can’t heal) and 3 upsides(can move freely within range, can hide, barriers don’t make difference on the damage) , so the problem with mercy lays on the range, damage increase % or like some said (cooldowns), she has low risk and high reward doing that, reducing the reward means more balance. Take zen by stance, the got high risk and high rewards in his orb, he needs keep line of sight and also that target needs be exposed (without any kind barrier or damage mitigation). So 3 downsides(line of sight, don’t have barriers in the way, bad mobility) to 3 upsides (damage amplification from all sources on target, can heal somebody and can attack).
Mercy most of the time was picked because her less risk and good rewards, also because her mobility and rez. So overall she have more “safer play and good rewards” against zen who have “riskier play and high rewards”. On “pick” perspective, most of the time the choice would be done on “safer play”. Because of that barriers dominated too much time.
About ashe, her abilities are not that well balanced at all. Her dynamite it’s too strong to be a ability, like rez. So higher cooldowns could solve the issue or just reducing the damage done like 75 explosion and 50 burning. At least all heroes can survive the blast without receiving any additional damage. Her scopped shots are ok, maybe less firerate or less ammo or both can solve(like I said) or just increase her damage and make her shots fewer and slower like widow and hanzo.