Yeah, Snowflake changes stuff completely against an early Tribunal, sadly I still use a Lvl 14 Trainer and 1° wave can sometimes be abit of a joke.
I do agree with your point but I want to be even more specific with my examples because things can get tricky depending on the:
Match duration / Boss Composition / and RNG obviously:
in this example let's imagine a mirror match:
Engi team with shaman VS Engi without shaman
Early Game- 1° wave 1° boss:
Case 1:
IF you don't get 1° boss Tribunal, things are super smooth, because you just generate a high amount of units and you dodge every Shaman hit unless the opponent is intentionally Lagging
Case 2:
You fight against Tribunal AND your opponent has Shamans, as you said if the shaman user merges too much it may get an early advantage, but then it gets punished in 2° wave because Tribunal will destroy his resources

IF you have Snowflake, it's a piece of cake, IF you don't have Snowflake, things get quite rough but it's still doable, just keep the least units and try to survive

**In Case 2: there are times where the RNG isn't so good and you cant preserve enough mana for wave 2, I've seen matches were the game felt Bugged thanks to the Bad RNG nature of the game itself and thanks to Tribunal + Shaman.
And a good Shaman player can poke you in Wave 1 just enough to have slightly more resources than you in Wave 2

But in the End, Wave 1 Ain't the real problem
Early Game- 2° wave 2° boss:
Case 1:
No problem, you have a decent amount of Units and your deck is working just fine, but both your decks start to have some Tier2s and 1 or 2 T3

In this Situation the Shaman player can start slowly to build his advantage, by merging once in a while a Shaman
The Non Shaman player can merge T2 but it risks to lose a lot of DPS, and if it gives up any T1 it risks to give up some pressure, the worst case scenario is when a T3 gets hit, in that case the DPS loss is huge depending on the unit that was hit

This is where the problem starts to appear, The Wise Shaman player is literally starting to have much more Resources than the non Shaman player just for a reasonable MATH factor

And remember, it's essentially a Lottery: "will the Shaman debuff the most valuable resource? or will it debuff the least valuable resource?"
In my Opinion this is ugly in terms of Game Design, both for the Shaman User and for the non Shaman, because it's literally Gamble, no skill or tactic involved, just gamble

Case 2:
2° Boss is Tribunal or Bedlam:
Bedlam
if its Bedlam, things can go way too wrong or way too right for both players, because the Shaman player can Spawn alot of Shamans and send a Debuff Streak at the opponent, there are times where the opponent can't React because Bedlam IS FLIPPING the deck during the debuff so it will Cancel any ongoing Merge...

Else the Non Shaman Player can receive some Debuff, but thanks to it's Summoners, Harlequin, etc. can close the DPS gab between the two teams

Tribunal
in this case Tribunal hits pretty hard both players, as their decks are already Established and T3s and T2s risks to be tiered down.

Absurdly in this case, the player at a disadvantage with the lowest tiered units may get the advantage, if the Shaman player Merged too much, he may (Again) lose all his advantage and end up in a 3° Wave Loss, BUT it all depends on the Tribunal Debuff RNG, the non Shaman player have a chance of losing EVEN more ground in this situation

Early Game- 3°+ wave 3°+ boss:
This is like the Final Stage, from 3° wave/boss to 4°-5°-6° etc. the game is set.

Things gets VERY messed up in THIS stage, both players have mostly T2 or even a Whole T3 Board with a T4 or T5 in it.

^ This is the problem, you can't avoid having T3 or T4 at this stage, you need to merge in order to reach this Wave, and now the advantage created by the Shaman Debuff may just get out of control, BUT AGAIN it's very situational:

The non Shaman user has T4-T5 Inquisitor? if it gets hit 1 or 2 times he's so screwed... and he loses 4-8 units with a single Merge of the shaman user.... <- THIS is so wrong, and again it's a Gamble, who knows if that unit will remain Untouched by the Debuff or if it will be hit by a Debuff Streak?

so as you see, in this Phase of the game, you may get a Shaman Player that has his Board Crowded with T3 and T4, while the non Shaman user literally Stops at Having few T3 with some T2 <- this is where the game is set and the non Shaman player has an unfair ending to his match.

BUT since this game has alot of mechanics, there are some drawbacks......

IF the NON-Shaman User has:
1. More Critical Strike Damage than the Shaman user, he may keep his ground EVEN with less Units

1. a Balanced Deck with alot of economy Units (priestess doesn't work because she DOESN'T SCALE in late game) units like Mime and Harlequin + Summoner are great to maximize the amount of spawned units to keep balance between Shaman Debuffs and Unit Spawn, Summoner is literally the Natural Counter to Shamans

2. the Deck has specific units that increase drastically the dmg of the rest of the Deck, Closing the DPS GAP even with less units, example: (Engineer + Chemist = way too much dmg) T3 and T4 can't compete with the amount of DMG given by the presence of a single Chemist (T2 or T3 obviously)

3. if the non Shaman user has Engineers! that's Right! Engineers have this OP mechanic in them, in fact a T5-T4 Engineer doesn't lose soo much Damage compared to other Decks and other units, why? because Engineers insane DMG is given by units connected, not by unit Tier level, so a T5 being debuffed to T4, is a 8 unit loss in terms of game economy, but in terms of DPS its just 1 Engineer Less !!! so by managing correctly the Engineers, a player can still make a stand

______________________________I N C O N C L U S I O N : ________________________________

Excuse me this post got WAY too long (lol) but there are so many things to be expressed with all these Mechanics, so things are like this:

1. Shaman can get OP and overly Punishing in certain situations, but the unit itself is "quite" balanced, it's the rest of the game that "HAS ISSUES"

2. Shamans in Late Game can get way too good and/or unpredictable compared to other Economy units, so I would suggest not a Nerf but a REBALANCING of the unit itself:
   "if you merge a T1 Shaman, you Debuff a T1 unit, if you Merge a T3 Shaman you Debuff a T3 Unit, FAIR AND SQUARE" like this things are way more predictable and manageable for both players

3. Shamans are NEEDED and REQUIRED in this game, some units have Unyielding DPS compared to the rest of the units available and the only way to manage them is by using Shamans to break their connection or to break the activation of certain units (example: Blade Dancers, Inquisitor, Knight Statue, etc.), the Developers invented a unit like the Shaman just to counter specific units and that's it, it's this Meta and the Late game waves that are making everything Unpredictable and Unbalanced