So as far as I understand, the main benefits of becoming Herald is:

1. Being half spirit enables you to shift between both forms. Edit: Also enables them to forge madra in order to regenerate quickly and greatly boosts their physical strength and speed.
2. Immortality (As in you no longer age).
3. You can throw out much larger techniques much faster (Yerin was able to throw out multiple fully formed techniques in succession, which she couldn't do before)
4. "Boundless" madra?

While the top three are nice, I don't see how they are big of a deal. You can get benefit #3 from diamond veins and I don't get how benefit 1 & 2 are helpful in combat.

Yet heralds are portrayed as being much stronger than archlords, so they must get something else, right? Yerin describes her madra as "boundless", but not necessarily of higher quality. Does this mean heralds simply have a much greater madra capacity (so more madra)? And this, combined with being able to cycle faster allows them to consistently throw out what would be fully charged and powered attacks from an archlord like a machine gun?

Is my understanding correct?

If my understanding is correct, it means that archlords are also stronger than we previously thought. Fury's attacks have been described as being able to blight forests and destroy entire cities. Logically speaking, this means Fury at archlord can do the same, but it would take him a lot longer to form the same attack, and he can't attack as often due to a lower madra capacity.

This means archlords are also capable of city destroying attacks, which is pretty cool. Since Archlord soulfire isn't any worse than that of a Herald like Fury, they can also fly around at supersonic speeds. This is consistent because we see the winter sage fly around as well.