I'm sure there are some items missing but running old OSX I'm seeing the issues creep up. From Apple services not working 100% to finding my favorite software unsupported. Safari doesn't even sync my bookmarks. I expect as Apple moves ahead, their other services won't be as smooth as iOS syncing with 10.13 or newer.
I had an old chromebook that is not supported and tried Elementary on it and it is working amazing. For 6 year old hardware, it works well connected to my old 23" Apple display and accessories. So I thought if I replaced the SSD in my Mac Pro with a new one and install Elementary OS, it should give me more years out of this 3,1. Running 10.11 is fine but finding software, it is going back to 10.13 mostly for support.