TBH, synology and qnap have awful track records when it comes to data integrity and cause a ton of issues even when using the software the "intended" way. SMB is a chatty protocol in general which makes it hard to troubleshoot, and because of default lack of write-through, you have a non-trivial chance of a situation where the SMB stack returns "this write was a-okay!" but in fact the data never landed on disk and only hit the cache.
My team works with a lot of clients for backups and we've had so many silent corruption cases with lousy synology and qnap boxes that we just don't support them when using SMB or NFS anymore; the built-in stack is just too unreliable. iSCSI is a bit better, but iSCSI isn't how a lot of people want their NAS to work, so it's always a tough discussion. General purpose servers with proper endpoints have always been better, but it's a hard sell to clients.
Directly-attached USB is marginally better in that you don't deal with network protocols, but a lot of companies cheap out on USB controllers and it's a challenge sometimes to convince clients that the on-board controller is the issue when single (small) file writes work without issue.
But long story short, I'd propose a small general purpose server (even Raspberry Pi!) over a QNAP/Synology any day of the week. The latter does everything simply but with excess mediocrity. From my point of view, they hit MVP across a dozen + items, but competency in none.
People relying on reliable backups building their own storage servers from scratch. Clearly this is universal sage advice that will certainly not end badly for the vast majority of people -_-.
I'm on my second Synology, the first was a cheapish 4-bay; now I'm using a much pricier 8-bay. This is for personal use, but the time and effort saved by using this over hand building has saved me factors more money. I do recommend NFS over SMB for the cheaper models as smbd is a lot more CPU heavy and transfers actually became CPU bound pretty quickly. I've never had data corruption issues in my 10ish years of use, but one subjective data point is pretty crap.
I don't imagine that there are many personal users who need/want ISCSI or other more raw block I/O protocols. As for businesses, off the shelf solutions in your requirements range are often available, and unless you're looking for very specific optimized workloads, there's probably a bit excessively expensive vendor solution that can meet those requirements.
My team works with a lot of clients for backups and we've had so many silent corruption cases with lousy synology and qnap boxes that we just don't support them when using SMB or NFS anymore; the built-in stack is just too unreliable. iSCSI is a bit better, but iSCSI isn't how a lot of people want their NAS to work, so it's always a tough discussion. General purpose servers with proper endpoints have always been better, but it's a hard sell to clients.
Directly-attached USB is marginally better in that you don't deal with network protocols, but a lot of companies cheap out on USB controllers and it's a challenge sometimes to convince clients that the on-board controller is the issue when single (small) file writes work without issue.
But long story short, I'd propose a small general purpose server (even Raspberry Pi!) over a QNAP/Synology any day of the week. The latter does everything simply but with excess mediocrity. From my point of view, they hit MVP across a dozen + items, but competency in none.