Honestly, so far if you can afford the up-front investment for the space you need, and the ongoing power costs, a NAS with a RAID array (or similar redundancy scheme) that can tolerate more than one drive failure at a time is probably the best long term archival storage. Spinning rust disks in my experience rarely completely fail without warning so you can usually catch and replace failing media before data loss occurs. Additionally if you don't, I've found that recovering data from failed HDD is also usually "easier" and "cheaper" for most values of both compared to other media storage (admittedly with no experience with recovering tape media)