This is the history of every upgrade for me. Always some weird problem that "no one has ever seen before". I had a conversation with someone who consults for Esri and says he's done 100's of upgrades with no problems. I am sure it's better when you know the answers to the ambiguous upgrader script questions in advance? I always do the upgrades off hours and that means there is never a support tech available to me.
At 10.8 every web app builder app stopped working and Esri was unable to help. While waiting for an answer ("we've never seen this before") we migrated all apps to the WAB Developer Edition to avoid any repeats of that in future upgrades.
At 10.9, one of the components (Data Store) never installed correctly and now I have to fix it before I can go to 11. (It functions but runs under the wrong ownership.) Spent hours with techs and they were not able to fix it so I gave up.
MOST of the time, Esri support is great. Sometimes? Useless. I guess human beings work there.
We have users who will be on ArcMap until it actually stops running. I am sure everyone does.
