Hello!
What kind of advantages has High Sierra compared to El Capitan?
I would say the three biggest immediate benefits of High Sierra over El Capitan are A.) the new APFS filesystem (which mostly benefits systems with SSDs), B.) active and continuing development on core technologies as well as base applications like Safari and some of the ones you mentioned, and C.) Safari will now block videos from autoplaying. Not sure if the latter feature is available on the El Capitan version of Safari.
Also, High Sierra introduces support for HEVC (High Efficiency Video Coding or H.265) the nominal heir apparent to AVC (or H.264). As more content is released in this new format, end users will benefit from smaller and quicker downloads.
I'm sure Apple has a list of new features in High Sierra on the marketing page on the corporate website. I upgraded from Sierra and I didn't notice many changes so I suppose most of them are incremental.
Moreover, third party developers eventually stop supporting older versions of the operating systems when they update their applications. It depends on the developer and of course whatever applications they write. We don't know what you do with your system, so it's impossible to make any sort of recommendation.
Are there any bugs in Mail, Calendar and Adress Book?
While I haven't encountered any, my assumption is that there are bugs in Mail, Calendar, Address Book and other applications.
All applications have bugs.
I haven't seen any showstoppers/dealbreakers, but I can't guarantee that you won't run into an inconvenience or annoyance.
Does it run well on a 2012 MacBook Pro Retina with 16 GB of RAM?
I don't know.
It runs well on my 2010 Mac mini with 8 GB of RAM as well as my 2013 MacBook Air with 8 GB RAM.
Heck, for me, just having Safari block autoplaying video was reason enough to upgrade from Sierra to High Sierra.
