I haven't unlocked Harmony TB yet; I'm seeing glowing reviews now but I wonder how their kit will age.
I've watched videos from a year ago when people were first unlocking Preservation TB, and players seemed to think they were amazing back then. Now they're a niche unit at best.
There will probably be something better at some point, but just ballparking some numbers:
You have just about 100% uptime on their ult, and therefore about 100% uptime on Super Break and +33% BE.
E4 gives everyone 15% of TB's BE, and you don't really need to care about anything other than BE and Spd, so if you get to 300 that's another +45%.
You also apply extra delay to broken enemies, so there's a Ruan Mei -like pseudo-sustain effect here.
As a stat on relics and stuff, BE is priced like Crit Damage. So the stat value of this 78% BE is like 78% CV. That's only a little less than you're getting from Sparkle's skill.
I haven't seen anyone produce the formula yet, but Super Break seems like a significant source of damage.
They're already a niche unit, because they want a break team, but in a break team this has to be pretty good.