On the contrary.
This is a coop PvE game, and the difficulty scaling is based on multipliers to enemy health, as well as spawn rates changes.
This means that things that are OP on Cata, or even on Cata 3 DWONS are multiplicatively OP on lower difficulties ; and that any nerf you apply to a weapon to bring it in line for higher difficulties only fractionally affects lower ones.
I think that “balancing for higher difficulties is bad” is a fallacy that stems from games like LoL that revolve around PvP and where there are quite noticeable meta shifts based on rank (*).
I’m not saying that lower difficulties shouldn’t be taken into account when balancing, because obviously they should and that is where the majority of the player base lies ; I am saying that Champion difficulty will be balanced if Cata is balanced, and Cata will be balanced if Cata 3 DWONS is too - and that is not true the other way around.
The higher the difficulty, the more precision and fine tuning is required to actually get the balance you want, and therefore the least impact it has on lower difficulties ; whereas balancing for recruit would be much coarser and have an amplified impact in higher difficulties.
For example : buffing a weapon’s damage for recruit to change a breakpoint from 3 to 2 hits has a disproportionate impact on Cata, reducing the same weapon’s breakpoint against the same enemy from 13 to 9 hits.
On the other hand, buffing the weapon’s breakpoint for Cata from 13 to 12 hits has less and less of an impact the lower you go in difficulty, down to none on recruit. Here’s a theoretical example table I put up really quickly to illustrate - I based it on an enemy with 20 HP and a weapon dealing 7 damage - the weapon’s buff for recruit balance is upped to 10, the one for Cata is upped to 8. The HP multipliers are the ones the game currently uses.
(*) There is an argument to be made for weapons like Greatswords, which shine brighter the higher the enemy density, and that does correlate with difficulty. That means that those weapons cannot be balanced around Champion because they will be absolutely OP on higher difficulties ; or they will have to be reworked in a way that would make them lose their “high density management” niche.
That being said, it isn’t because they shine brighter in higher difficulties that they’re not good in lower, other options are more fitting is all. They’re still perfectly usable and balanced, albeit a bit boring - which isn’t a balance issue as much as a design one.
Except for IB and Zealot, all classes are pretty squishy… Even “tanky” classes like FK get punished hard for getting hit ; ultimately tankiness isn’t really a metric. A character is only as tanky as the player can dodge/block hits and as they can generate THP.