I upgraded my Google Nexus 5X to Android 8 a while ago. It now includes several updates and is at 8.1. What a mess.
Alarms go off late sometimes (I think mostly when the phone is in sleep mode).
SSHelper, a very useful app that runs a secure server, doesn't work. Apparently Android 8 aggressively shuts down background tasks to save battery power. Why doesn't it let me exempt programs?
And battery usage doesn't appear any different than with Android 7.
Sometimes, when I turn on my phone, it comes up in Google Play. Why?
Insets are a useless gimmick. It was so annoying with Maps that I had to disable it. (I do appreciate the disable option.)
I haven't found a good use for split screen, so that's a waste.
Dolphin browser, often leaves the keyboard on top of web pages, about 50% transparent. You can't get rid of it. I don't know if this is related to Android 8, but that's when it started doing this. I was using Dolphin because was dramatically faster than a previous version of Firefox. Chrome incorrectly renders some tables, making it useless for some web pages. Now it's back to Firefox.
I finally gave up waiting for Android Chrome to be fixed. The problem is multiple lines separated by line breaks in a table cell - these caused the font size to go crazy. I replaced the multiple lines with a single column table. Problem fixed. What a pain. I was set to start using Chrome. It has a new feature - a news feed on the home page. And an annoying tab menu. So I'm using Firefox, but my contra dance links work with Chrome now!
Of course they still haven't fixed the problem that when you use an app to turn off the screen, the fingerprint reader won't turn the phone back on. That started with Android 7.
There is no reasonable way to go back to 7.
I suggest another "law" related to computers (including phones, probably tablets but I have found no use for tablets), along the line of Moore's law about computer processing power. Every year, consumer operating systems add features and bugs/incompatibilities at about the same rate. This makes them less and less useful. Eventually they will all degrade to the point of being unable to handle basic features - screen, phone, internet. Then it's back to dedicated devices - flip phone, calculator, music player, ...