I thought I would chime in with some personal observations regarding modern and vintage nibs.
As mentioned, each nib writes differently. I find that personally, sometimes a nib forces me to write a certain way. A stiff nib forces me to slow down and use a more controlled movement, a flexy nib forces me to speed up and use a lighter hand. This affects my writing style especially if I did not have exposure to different kinds of nibs. Not sure if this is true for anyone else. I use a certain flexy nib for engrossers, a stiff nib for copperplate, a super flexy one for big lettering, and a tiny one for small writing, etc.
Therefore it is hard to quantify what makes a 'good' nib. If I had used nikko Gs all my life and developed a very controlled style, I would dismiss a brause rose right away because it doesn't suit my hand. I have a collection of many modern and vintage nibs, and have dismissed many of them as 'bad.' But over time I would try them again with new knowledge and realized they were actually pretty good, I was just using them in a way that doesn't suit that particular nib.
I don't think it is fair to call a nib good just by its ability to create a fine (thin) line. Both modern and vintage nibs have the ability to create thin lines, it's just a matter of a light hand. What does matter to me, and what causes me to call a nib GOOD is a nib's ability to create a
quality line: smooth, snappy and responsive tines and the quality of the metal and pleasure in writing. I can probably write well with a low quality nib, but I wouldn't enjoy it, the metal would be scratchy, unresponsive, and a nightmare to get a good line out of. A good quality nib will be a pleasure to write with and add to my ability to coax good writing out of it.
It is true that vintage nibs go through higher quality control than modern nibs. I don't know what cross-grinding does to a nib, but it is an indication that this nib is probably much better quality than its modern counterparts, due to how much attention was given to its production. Which is why I tend to call vintage nibs good, as they are much more likely to be using good steel and quality control. However there are some modern nibs which are also very good quality, and even be comparable to vintage nibs. But some (if not most) of modern nibs are terrible quality indeed.
I find it hard to explain why some of us are so obsessive about vintage nibs. I find it is quite personal, similar to a search for good wine. To most, a regular bottle of wine from any shelf will suffice for a party (that is perfectly fine). But in our obsessive context, it is also about how the wine will pair with food, what region it is from, how rainy it was the year it was harvested, how it was stored, the age of the barrel used etc. Obsessive, but very personal and hard to explain, and comes from years of accumulated experience! Just because a connoisseur prefers a certain wine from 1910 doesn't mean it will appeal to everybody's palate (script).
Alas, a layman hears about the connoiseur's obsession with the 1910 wine and think, it must be good! And so the hunt begins for these rare items and prices go up whether or not the buyer appreciates it. Like a snowball effect, the notoriety and rarity spreads and one starts to develop an unusually high expectation, when in fact it is simply just one person's opinion and liking for the wine. Personally, I love and use vintage nibs as they are pleasurable to write with, but modern nibs are perfectly acceptable! Some amazing calligraphers today use modern nibs exclusively and create astounding work with them.
However, I thoroughly agree that Musselmans are terrible and not worth its box. If you ever come across a box of it I would like to offer you TWO boxes of modern 303s in exchange for those rubbish nibs!