Well, I haven't got to fix ''ghosting'' in my recordings under direct-to-disc mode no matter what I've tried so far.
I matched frame intervals with actual frames in my cameras, lowered output framerate in general, increased received buffer for every single camera in BI, unchecked (and re-checked) HW acceleration option, played with various resolutions my cameras outputting, even tried different type of destination drives to do the recording (USB 2.0 HDD, USB 3.0 HDD, SSD (both main system's one and a secondary SATA) as well as a USB thumb drive) to no avail.
I also deactivated Noise Reduction in the configuration of my IP cameras, although the problem has obviously nothing to do with the cameras themselves and their stream quality, rather than with the recordings in BI.
Now that I've opted to ''re-encode'' the videos being recorded, the problem seems to have gone away. However, CPU usage is much higher now..