I have an mdisc drive, and a few mdisc DVDs I haven’t burned yet. Someone on/r/datahorder noticed they don’t sell real mdiscs anymore: https://news.ycombinator.com/item?id=33593967
People buying M-Disc's will not notice that they've been sold BluRay discs for decades when they start to decay some day. It's a malfeasance which will only be discovered in the distant future once the discs start decaying, and will be someone else's problem by then. Meanwhile whatever product manager will have made their promotions for cost cutting and increasing revenue.
tl;dr only buy the M-Disc's which are rated for 4x write speed.