Harmony – Here's what's wrong:
There's not actually very many harmony cards.
Harmony is very slow tempo. Other comparable archetypes (Assimilate, Thrive) either allow you to play multiple engines in one turn (Nekkers, Endrega Larvae) or proc the condition multiple times (Artorius Vigo, Braathens).
The core bronzes are massively powercrept.
Half-Elf Hunter and the ST stratagem have antisynergy with Harmony. When the Half-Elf hunter or Neophyte is removed, the other body spawned prevents Harmony from being triggered again by elves. On a side note, why does ST have the worst Stratagem in the game?
The order of cards feels very unnatural. You want to play Dryad Rangers early to take advantage of their harmony tag, but you also want to play them late to take advantage of their poison. Not every card has to be perfectly synergistic, but sequencing on a bronze card should never be so disgustingly complicated.
Besides Waters, Barnabas and Percival, there's not really any decent gold harmony support cards. Saskia is a complete joke because she can't trigger harmony twice in one turn. Compared to Braathens, she's more expensive, less versatile, and understatted. Having her spawn a Young Dryad also makes no sense because that's an engine from another archetype.
Harmony – How to fix:
In my opinion, Harmony should be very engine heavy and have an oppressive amount of points in a long round, like a Koschey deck. At the same time, their engines should (mostly) have a pretty high floor so they do alright in a short round too. Their main downsides should be the devotion implied by the Harmony tag and that their control is limited to poisons, natures rebukes, and maybe one of the 6p locks.
Here are some ideas:
Buff waters. It's basically just an expensive Endrega Larvae plus 4 tempo. There's no reason it should cost 12 provisions, even with echo. The only reason it was ever costed so high was because of double waters, but that hasn't been around for well over a year now. At least give waters something like "Devotion: Echo"
Let Saskia create and play a bronze Scoia'tael unit from your starting deck. Make her more like Braathens.
Give Percival Schuttenbach immunity or something. This would be a huge buff, so maybe also give him +2 provs. However, the immunity would really help your more fragile engines trade up versus removal. (Also, I'm tired of other +2 per turn bronzes like Glynnis not being played because they just die to tall punish. These cards need to be discussed further.)
Give Harmony some devotion payoffs. The way the ability is worded almost always shoehorns it into devotion anyway. Remember I said it should be a greedy archetype? Devotion is a good way to enforce greed.
Harmony honestly needs something like Nekkers or Endrega Larvae to truly shine. Maybe a 2-power Naiad card for 5 provs that spawns a copy in the same row, like Nekkers?
Give Harmony to Figgis so ST doesn't have the worst defender in the game.
Harmony also benefits from non-harmony engines with interesting tags Cat Witchers and Hawker Smugglers. Having more of these cards would help harmony.
Gezras is going to be a staple in most modern harmony decks. His downside is that he's a long round payoff card, so including him hurts the short round potential.
There needs to be some sort of logic to what gets played when. Obviously you play Dryads first, usually followed by other harmony units, followed by non-harmony engines, followed by harmony-related finishers. Right now, Barnabas Beckenbaur is the only finisher.
Trained Hawk has some nice synergy with Cat Witcher. Would be nice to see that shine, if Hawk's stats weren't complete garbage.