I've never used Synology NASes, but one feature that Synology NASes have that I found interesting and unusual was their ability to glue drives of different sizes together, what they called "hybrid RAID". No single-drive point of failure, but permits for using drives of different size without wasting a ton of space.
While this isn't, internally, all that technically complicated to do --- internally, I understand that it's just slicing the disks up into a bunch of Linux LVM logical volumes and merging those into an md device, keeping in mind where the logical volumes are located. You can build something like it on a Linux machine with LVM and md. However, it isn't something that other NASes that I've seen provided.
Last I looked, Unraid couldn't do it, and QNAP couldn't do it.
Based on a quick search, it sounds like Unraid still can't do that.