On 10.13.x to 10.14.1 need disable IOGVACodec otherwise system will very unstable.
On 10.14.2, use native AppleGVA (HEVC encoding is really fast, as fast as IQSV H264) and do not disable IOGVACodec, unlike 10.13.x to 10.14.1, on 10.14.2 disable IOGVACodec will cause all H264/HEVC encoder/decoder failure, same as DRM support, DRM only work when IOGVACodec is enabled on 10.14.2.
On 10.13.x use SMBIOS iMac14,2 is better, very stable.
On 10.14.2 use SMBIOS iMacPro 1.1 is better but need lilu + whatevergreen, iMac14,2 + novpa.kext work but unstable (10.14.x with IOGVACodec enabled), tested with different combinations (SMBIOS,patches,CPUs,Mobos,dummy kext), recently HW H264 encoding never work propertly from 10.13.x to 10.14.2, will cause system freeze.
When IOGVACodec is enable, system may not stable due to apps compatible issue.