...I respectfully disagree with generalizations like that - which I see repeated often enough on the internet. It is true that SLI/NVLink support in some games as well as scaling can be issues, but I run four machines (2x 1080P, 2x 4K) which all have SLI or NVLink, and I have encountered few problems, at least in the games I play. I add though that some of these machines are also used for productivity with their dual cards, so my approach - and experience in tuning mGPU- is different to begin with.
More importantly, NVidia quietly introduced drivers that support NVlink CFR only last fall (as opposed to just AFR). This is so because post-Ampere, mGPUs will play a much bigger role (similar to AMD's multi-chiplet approach about to be joined by Intel next gen CPUs.). NVidia would not introduce mGPU CFR w/o a reason, and I put a related thread up on this forum > here.
I am not suggesting that SLI/NVLink is problem free or enjoys support everywhere, but it certainly is 'not dead', and mGPU setup fans can particularly look forward to next gen. But even with current gens, you already saw the above NVLink RDR2 video links. Now, here is a Metro Exodus one @ 4K / max settings comparing a single Titan RTX vs dual Titan RTX (on CFR), including w/ frame times. In general, NVLink (AFR, CFR) may not scale perfectly, but if it means an extra 20-30 FPS in 4K to put the game at or above 60 FPS instead of 30 FPS or below, I'll take it.