Many Macs do support Hardware Encoding but only those listed below will support hardware encoding in 1080p60 in real-time.

iMac

• iMac13,1 (3rd gen): iMac (21.5-inch, Late 2012)
• iMac13,2 (3rd gen): iMac (27-inch, Late 2012)
• iMac14,2: iMac (27-inch, Late 2013)
• iMac14,3: iMac (21.5-inch, Late 2013)
• iMac15,1: iMac (Retina 5K, 27-inch, Late 2014 or Mid 2015)
• iMac17,1: iMac (Retina 5K, 27-inch, Late 2015)