Answer
Macintosh, Linux, Unix, and a dozen other devices and Operating Systems are hard to identify. If the Operating System is only determined by ICMP packets and not SMB or SNMP, the chance that the device will be correctly identified is reduced. This is due to the fact that most of these devices have a lot of common characteristics.
For more information on how GFI LanGuard determines what Operating System is running on a device, see How does GFI LanGuard determine the Operating System running on a device?
NOTE: GFI LanGuard supports only SNMPv1 and SNMPv2c. SNMPv3 and SNMP over TLS/DTLS are not supported.