As I wrote above. It’s not about speed, it’s about price and mostly availability. When I look at 2,5" at my country’s biggest retailer, there’s not really much to chose from and the number of available offerings are more or less shrinking. That’s not really the case with M.2 which seems to be “new shit” everyone wants so there’s plenty of options. And even if I stayed with “trusty WD Red” it’d still cost less to buy M.2…

Another benefit (for me) is its form factor. I don’t have a lot of space. Classic 2-bay NAS size is “perfect” for me, apart from its bay limitation. That’s what’s so tempting on Asustor. Either Nimbustor with 4xM.2 or even Flashstor with just 6xM.2 are quite a small devices (compared to what regular 6-bay would be) which is a big plus for me.