The floppy interface for 5.25 is largely the same as 3.5. The data rates are the same. The only functional difference is fewer sectors per track because of the faster motor speed. That's all accounted for with a FDC interface controlled by a BIOS that knows what each drive is. A USB FDC bridge is stuck on its firmware and has no way to adapt to alternate track layouts it isn't designed to handle.
Comments
The floppy interface for 5.25 is largely the same as 3.5. The data rates are the same. The only functional difference is fewer sectors per track because of the faster motor speed. That's all accounted for with a FDC interface controlled by a BIOS that knows what each drive is. A USB FDC bridge is stuck on its firmware and has no way to adapt to alternate track layouts it isn't designed to handle.