I have no idea. I've honestly never heard of subliminal until just recently (yesterday) and I've never really tried it out.
But on first glance it seem like subliminal is more for developers in general as it provide apis to use it, it also have a bunch more providers than what SubSync currently have and most likely a much better way for guessing which subtitle is correct. It also seem to focus more on individual subtitles rather than trying to batch download them unlike SubSync. Although they do have support for downloading for a whole folder. How well that works I have no idea.
In the end, Subliminal have been in development for over 5 years and has been thoroughly tested.
Whereas SubSync was a weekend project I made 2 weeks ago and never meant to compete with anything existing. I only knew I had a problem I needed to solve and wanted to share it with everyone :-)
TL;DR: I don't think SubSync offers anything unique over Subliminal at this moment. And if you're already used to using Subliminal you should keep doing so if that works good for you. :-)
Actually, one thing. The idea with SubSync is that you keep it running in the background and as soon as a new video file is added to your library it does look for a subtitle to download.
I'm not sure if subliminal has that or if you manually have to tell it to look.
One reason could be that you don't use Windows. Glancing at SubSync's GitHub repository I see it seems to be a Windows project, whereas subliminal runs anywhere Python does.
Its luckily a .net core 2.0 application so it is possible to run on mac and linux. :-) not completely hasslefree though. As you would need to install the required runtime for it.
Is python "natively" supported on those platforms or do you have to install some prerequisites for it to work? If so, it's just as "hard" to run this as Subliminal.
Yes, python is included, though I think macOS uses python 2.x (though people using cli tools will often have the latest). I’m guessing a google search could tell you what is bundled for macOS and various Linux distros.
But on first glance it seem like subliminal is more for developers in general as it provide apis to use it, it also have a bunch more providers than what SubSync currently have and most likely a much better way for guessing which subtitle is correct. It also seem to focus more on individual subtitles rather than trying to batch download them unlike SubSync. Although they do have support for downloading for a whole folder. How well that works I have no idea.
In the end, Subliminal have been in development for over 5 years and has been thoroughly tested. Whereas SubSync was a weekend project I made 2 weeks ago and never meant to compete with anything existing. I only knew I had a problem I needed to solve and wanted to share it with everyone :-)
TL;DR: I don't think SubSync offers anything unique over Subliminal at this moment. And if you're already used to using Subliminal you should keep doing so if that works good for you. :-)