Weapons with short CDs are absolutely crucial in this game because damage buffs primarily take effect by increasing the damage of a single hit.
From this perspective, the rusty pistol is superior to the Rusty cutter, especially since the Rusty series' gameplay centers around buff management.
Below is my rusty build beating the final boss. I lost no life overall. My base health was 490 (625 after enhancement), yet I still only barely managed to win with 16 health remaining.
I can only say that rusty series still lacks some powerful equipment, particularly in the "loot superior items". I feel rusty are somewhat isolated and don't synergize with other builds like Fire and Shock.
Additionally, the rusty shield is severely underwhelming. Like the shell loader, it's the only shield type in that series, but it's practically useless.