I agree that aluminum and steel need different inserts, however a general grade steel insert will work fine for stainless steel and cast iron. It may not be optimized for it, but for a hobbyist or job shop it isn't realistic to stock specialized inserts for each material as it's hard enough to get the all the inserts you need for each profile.
Figure you need round nose and square tip inserts in both polished for aluminum and molded for steel. That's 4 packs of inserts right there and will run you roughly $400 for authentic name brand inserts. That's out of my league as a hobbyist, and I suspect many others.
Yes, my goal for getting the Korloy is due to it being highly copied by Chinese manufacturers. That way you can buy name brand inserts from Korloy if you feel you need quality/consistency, or stock up on a variety of grades/profiles from the Chinese, which have worked very well for me. I'm know they aren't as good as the name brand stuff, but they work well and allow me to get a variety of styles. I've had bad luck in the past with Chinese holders, they were soft, pockets weren't machined square so there were gaps under the insert, etc. So I wanted a quality holder that had cheap inserts available.
As for different grades and profiles here are some examples.
General steel work
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Polished for aluminum, also right and left version with angled tips to help leave the knub on the side you want when parting
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Round nose for general steel work
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Round nose polished for aluminum
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Square nose for hardened materials
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Cubic boron nitride (CBN) round and square nose for hard materials
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You could buy all those for less than one pack of 10 inserts from a name brand tool supplier.
I used AliExpress as it's easy to find all the options. You can buy all those from US places like eBay and amazon as well, but you'll pay more for the same thing.
Im not arguing that the Walter and others are not good options, but you're shoe horned into paying premium prices for proprietary inserts. The benefit the Korloy has is that it's been copied by many manufacturers. They have different grades, you just have to learn how to read the broken English.