The thing is with 4K and this subject in particular is that with DP1.2 and effectively DP1.2a, you're using all the bandwidth. Hence my question to Robert as if you're looking for 4K, especially in a few months then at 4K I'm not sure how the use of adaptive sync will effect bandwidth. There is already a lot of issues with certain certified cables and non cert ones for this very reason
4K doesn't use all of the DP1.2a bandwidth, just most of it. There's no issue, here.
The cabling issues you're referring to have everything to do with cable quality: are they designed well enough to accommodate the high signaling rates of DP1.2 HBR2? Many shoddy cables are not. But that's not a bandwidth issue of the spec.