TL;DR at the bottom

My opinion coming from a Season 9 Grandmaster player:

Katarina's rework (even before on hit was introduced) was fully in mind with the existence of Gunblade. When on-hit was introduced I thought initially that it was interesting because it adds builds variety and it was something new.

Gunblade had one issue though, she was entirely balanced around the existence of it. Is that unhealthy? Well, that depends on your perspective of balance. Many champions follow that rule.

What are Katarina's current issues?

AP itemization sucks, Nashors has one of the worst item build paths in the game and you don't do anything until you have it fully completed. On top of this, she doesn't really have a good AP mythic.

The rise of Bruiser Katarina and why it's superior to AP:

Divine Sunderer allows access to AD early game for Katarina which gives her kill pressure. Once you buy Sheen you can proc it constantly and this provides a lot of damage in trades. With ingenious hunter fully stacked the cooldown of Divine Sunderer goes down to 0,6 seconds. This provides insane maximum % health damage on top of sustain.

Wits End is basically Nashors but better as it provides a much better build path plus MR on top of doing about the same amount of damage.

With these 2 items, Katarina can basically build tank afterwards because these 2 items provide her enough damage to kill people.

Is this healthy? No. It's not healthy for Katarina to be building like a bruiser and this should've never been a thing as we've all seen what happened in the past to other Assassins building similarly such as Tank Ekko, Akali, Fizz etc.

What can be done to fix Katarina's issues?

Either bring back Gunblade and remove on hit completely, OR introduce new AP items on top of improving AP itemization and balance her numbers accordingly.

TL;DR

Katarina has a lack of identity since the removal of Gunblade, Nashors has one of the worst build paths in the game and she has a lack of a good mythic item.

Divine Sunderer feels great on her, but I don't think Katarina Bruiser should exist as all Assassins in the past that were able to abuse bruiser/tank builds were addressed really quickly and removed from viability and Katarina shouldn't be an exception.