However, I've seen Reaping Strike's impact on play aplenty. When interfacing with the minion rules it is irrelevant; minions can't die on a miss. However, this situation provokes the premise movement from "but unbalanced" to "but realistic simulation." 4e provides immunity to such an issue via those minion rules and the fact that proportionately, the HP total:Str damage ratio provides thematics (relentless berserker et al) and tactical overhead (and some fun with build synergy), rather than raw power on a per deployment of Reaping Strike basis. 5e has neither of these in place that I'm aware of so At-Will damage on a miss could be a problem (i'm not convinced yet as I'd need to see the math) with low HP/unit and no minion immunization to damage on a miss.
First, I've played a lot of 4E - epic levels and all that, and am still in a regular game.
Second, I am not a fan of damage on a miss powers/abilities.
That being said, I have the following comments
The powers with damage on a miss in 4E, IMO, did have one definite downside with the way the game played at the table. That downside was how they interacted (or failed to interact) with minions. On numerous occaisions a DoaM power would kill the BBEG and leave it's minions standing - as we got higher levels this happened more often because that half damage became pretty significant.
However, this problem could have been mostly mitigated because of the multiple defenses everyone had. For example, for a DoaM power that targeted AC, instead of it being
Miss: half damage it could have been
Miss: If the attack would have hit the target's Reflex Defense it does half damage.
Doing something similar in 5E is how I think this particular ability should be handled - so it's not the character can never not fail to do damage but the character has a fall back that let's him do damage when he would normally miss.
If the designers choose not to take that path I hope they don't make it core and instead put it in some tactical module. The reason I feel this way is that I much prefer to make house rules that add something to the game instead of take something away from the game.