People are panicing about the new damage system and it appears they don't even take a moment to realize that it's not as complicated as it seems.
So I decided to present it in the simpliest way I could.
Here I state my opinions on the current system and we all know opinions on the Internet are the worst thing.
Now, I realize where all this confusion might come from. Codex doesn't really make a good job at explaining things. On the wiki, people see four columns and panic immediately, as if they were supposed to remember entire tables (spoilers: You only need to remember the ones with highest +%).
Corpus crewmen and their shields have separate resistances and they are being damaged separately. To destroy a crewman's shield quickly you use magnetism, but then to deal the most damage to the naked crewman you'd need to switch to viral. This is why shields are listed as a separate thing from their users.
But for Grineer, their flesh and armor are damaged simultaneously meaning both flesh and armor weaknesses are applied at the same time. That means they could be categorized as "ferrite armored" and "alloy armored" instead of listing their flesh as a separate thing. The fact that grineer flesh is listed separately only makes the whole thing seem more complex than it really is.
As for light infested, the damage has been made quite nicely. Light infested are all weak to the same elements, but to a different degree. They also have different resistances. This makes barely any difference but it's a nice touch. They're mutants after all, why would they all be the same?
The only thing that I really don't understand are the ancients. Most of the ancients have the same resistances, aside from healers. What's so special about healers not to make them weak to corrosion and blast, and use radiation instead? It's rather weird to make player make a different build against a single enemy type.
In short, I think this system is quite nice aside from that infested sinew which seems unnecessary. Honestly I'd rather leave it this way because I'm afraid further updates might mess this up.