Every solution has its special advantages and disadvantages.
Synology for example is basically an average Linux system, paired with some special raid options and a quite low quality hardware if you compare the price. Storag features are not as state of the art compared to state of the art solutions like Netapp or ZFS. But Synolgy add a bunch of extra non storage apps like media, foto or cloud applications and the are maintained in a superiour manner.
Qnap is similar but with a little more focus on hardware and a little less on extras.
The alternative (igoring cheap desktop hardware) are server systems from Dell, HP, HP, Lenovo or Supermicro with a far higher hardware quality and much more options. You can pair such a hardware with any software solution. Only remain aligned with the typical suggestions for a good ZFS filer. If this fits, every standard OS solution will fit ex see
https://napp-it.org/doc/downloads/napp-it_build_examples.pdf
ZFS is the storage champion nowadays, the state of the art filer solution and the only free system comparable to the real high end systems ex NetApp.For ZFS you also have the options
Solaris, where it comes from, either a commercial Oracle Solaris with native ZFS or the free Solaris forks like OmniOS or OpenIndiana. They offer the best of all ZFS/OS integration, the kernelbased SMB server with best Windows compatibility and Comstar, a enterprise iSCSI stack. Perfect if you mainly look for a trouble free ZFS server for FC/iSCSI, NFS and SMB with very newest ZFS features like encryption or special vdevs where "Solarish just works out of the box on a minimalistic setup". If you are looking for a special media app ex Plex or integrated apps, you propably won't find. Virtualisation of Linux or Windows for them is then the answer.
Linux offers everything. Hard to find a hardware or software that you cannot use on Linux. Problem is often complexity and reliability on ZFS, software or app updates (this is why Synology is there). Sometimes more an adventure game than a "it just works" solution.
Free-BSD ex with FreeNAS is somewhere between. Very good ZFS integration (although currently outdated as all new ZFS features are missing, will change propably mid 2020) but with many apps that you can install ontop of FreeNAS - but far away from the trouble-free Synology experience but with superiour storage features over Synology.