Having used SLI in the past I will say that games from DX9 through DX11 maybe 40% or so support SLI/CFX natively while DX12 does it different as seen in Ashes of the Singularity.
I have an X570 board but the model I selected is designed for one card. Right now the card is an RX 480 which has 8GB of memory and 36CU. It's not hurting.
So depending on the range of games you have it will be hit and miss as to working.
I recall a person here with a HD 7990 which is a dual GPU card who also ran into mixed support.