M.2 in most instances is a more elegant solution. A PCI-E M.2 adapter card costs less and takes up less space. Add to that the availability of small form factor solutions and M.2 is a better format. Sure, I'd probably rather have this for older 2.5" drives, but not many people have those just sitting around, but a USB adapter is a lot cheaper. The problem with this is that if doesn't have a small form factor version. If it did it'd be AMAZING for smaller computers that are limited in the drives they can hold by their size, but M.2 works there too, in new systems anyways.
Yes, there are a few solutions where this makes more sense, especially if we are talking about adding onto older hardware, but for the price of this plus drive, a 1TB M.2 makes more sense than this plus a 1TB 2.5" HDD. The M.2 solution would be a lot faster too. With 2TB drives this solution is good, for anything 1TB or smaller M.2 makes a lot more sense... and if we are honest there aren't many, if any, 2.5" x 9.5mm drives over 2TB.