I kept my Macs on Mountain Lion until a few months ago for compatibility with some expensive legacy apps. It would have cost around $7000 to update them all and I can't justify that since I'm retired and only need them occasionally. But I upgraded to Sierra and was kind of surprised to see that Final Cut Pro 6 still worked. My very old Photoshop CS3 also works, but I needed to install a legacy version of Java. It does have one annoying issue though, it can't overwrite an existing file, I have to change the name every time I save. Ended up getting a cheap copy of Photoshop Elements that can do most of what I need.
Logic 9 seems to work fine, but I've read about various problems on Sierra, evidently it can crash when you try to bounce. I updated to Logic X because I didn't want to deal with that and the cost was reasonable. Also upgraded to Final Cut Pro X and the new versions of Compressor and Motion. Am getting up to full speed now and they are big improvements over the legacy versions .
Microsoft Office 2008 seems to work fine but for some reason it takes a long time for the program to exit when you quit. Filemaker Pro 11 runs fine.
My old versions of Strata 3d cx and Vue Infinite seem to work but I haven't done much with them. My biggest problem was a CAD program, VectorWorks 2008. It just crashes and would cost me around $2500 to update. I have my quad mini setup to dual boot into Mountain Lion for the times when I need that. I did the free trial of AutoCAD LT and it would be a reasonably priced alternative if needed.
I'm sure all this old stuff that is working on Sierra will break on High Sierra though. Am not going to be in any rush to upgrade to that.
