Some copy protection systems relied on using subtle timing errors to create disks that would read as "corrupt" by standard system software. Replicating this behavior in an emulator would be a major undertaking as well as being hard to reproduce in an image format.
I was never very advanced at cracking, but I do remember back on the C64 I encountered a number of games that had corrupt/unreadable sectors on the disk. Many copy programs would halt when they encountered these errors meaning you couldn't copy the disk. I had one program (no idea what it was called anymore) that would skip the errors and write everything else. To beat the copy protection, I would open the sector with the read error on the original disk, swap out for my mostly complete copy, and then write the disk error to that sector. I always thought it was kind of weird that I could write an unreadable error, but it seemed to work most of the time.
I think the ultimate problem with emulation is the disk image: how do you image a disk with these subtle timing errors? The default behavior of the software operating the drive isn't aware of half tracks; every vendor uses different tricks so you don't know which 'timing errors' to activate; etc.
It's a USB floppy controller which allows you to image the raw magnetic state of the disk (subject to head step and width). It allows you to record the disk and then try to figure out what the format is. It's good enough to allow you to read CLV disks, such as Mac floppies, from an image taken from a CAV drive, such as a PC one. I don't know whether it supports half tracks --- probably --- but it will certainly support all the weird timing error tricks.
It's not what I would call user friendly but once you've figured it out the results are magical. I was able to image an ancient BBC Micro floppy disk once and then spend ages figuring out how to parse the sector layout and encoding density working with just the image. Not only does this avoid having to keep working with the fragile disk, but it's way faster!
