basically it crashes no mater what I do... I tried a fresh instance of blender and still the same thing, it happens when I mess with normals or displacement settings. not sure really but I do hope one day I can use my Vega pro... one day..
here follows the non emotional information for y'all devs to have fun with, thanks for all the hard work, very cool to see you guys taking on the industry. very happy to see.
Model: iMac19,1, BootROM 1037.120.79.0.0, 8 processors, 8-Core Intel Core i9, 3.6 GHz, 40 GB, SMC 2.46f13
Graphics: kHW_AMDRadeonProVega48Item, Radeon Pro Vega 48, spdisplays_pcie_device, 8 GB
crash when switch viewport to rendered view with displacement mapped PBR texture applied (no background HDR, No other settings changed).
Termination Signal: Segmentation fault: 11
Termination Reason: Namespace SIGNAL, Code 0xb
Terminating Process: Blender 
Application Specific Information:
objc: GFX9_MtlComputeCmdEncoder object 0x7fa6ad3a3600 overreleased while already deallocating; break on objc_overrelease_during_dealloc_error to debug
Full crash report included as .txt