I took over the network where I work 5 years ago, once I had settled in I realised we needed storage and backup space so after extensive research the obvious choice was Synology DS412+ for a network of 25-30 users, with 4x4TB WD Red. What a NAS!! Time passed and the network grew to 50 users, enter Synology DS1815+ filled with 6 x 4TB Toshiba x300 and 2 x Kingston 240GB KC400 drives. Got to mention here the Toshiba X300 is the best HDD I've used. A lot of iSCSI LUN usage, plus central file backup, usual stuff. When this was hit by bug, we thought we should update, it was going to be temporary, but now our Synology DS1019+, has won our hearts. Must be the NVME cache (still using the same Toshiba X300 drives from 1815). The DS1815 has since become a testbed for many projects.
As company has grown, we have expanded to other countries, our Bulgarian plant has DS416slim aside from choosing it have not really been involved with this one. Our Polish factory was part of a multinational company before, with MPLS connection and remote servers which were not available to us, so needed a rapid solution. Step in a pair of Synology DS1817+ loaded with 6 x 4TB Toshiba HDD and 2 x Samsung 850 evo 250GB SSD's apiece. These have been a showacase to the power of these NAS. HA cluster running domain for 35 users (credit goes to sieberta from the other community for AD setup), it has been easier to maintain than a windows domain, 2 years in no real issues. I setup iredmail server in UK for their factory, by the time we came to migrate it there was 150GB of email data to move, how did I do it - rsync from NAS to NAS! Easy. It is a multi WAN network running through Watchguard Firebox, loads of policy based routing and it works day and night. Hyper backup of data to 2tb external SSD over eSATA on 5 night drive rotation, awesome. Considering the NAS were £1700 the HDD were £1200 the SSD were £2300 (5 x 2TB for external and 4 x 250GB SSD cache), £5200 (+ VAT) was spent, it is money well spent. The equivalent windows server hardware and licensing to do the same job, with HA would cost so much more.
As you may have guessed, strictly a business user, so uptime is key for me. Not interested in non-business stuff, but am always looking for using the NAS anyway I can to make my day easier at work.
I'm glad I found the real synology community, those 'copied from the manual' solutions at the other place were about as much use as a chocolate teapot!