mGPU is better than old Crossfire/SLI, because you don’t need two identical GPUs - and the GPUs are not connected physically between them -, instead you can use different GPUs (years, vendors, models), and you can gain some performance with your old GPUs (it is not the same as Crossfire/SLI, when they said that gonna be ‘double performance’)
I found this presentation from AMD (2017)