I had no intention to upgrade my 2017 iMac from Mojave as it was working well and I saw no advantage for my workflows in doing so. Well that changed. Current products I like no longer supported Mojave and I was forced to either use cloud versions or upgrade to at least Catalina…I chose the upgrade but decided to go all the way. I had a surprisingly to me pleasant upgrade experience. After ensuring my Time Machine backups were solid and restoring test things actually worked properly, I used Techtool Pro to create a volume clone on a separate hard drive. I deleted a few apps that I no longer used, cleaned out caches and other obvious debris, emptied the downloads folder, and took out the trash. Then, in flagrant violation of good practices everywhere, I simply let my 2017 Mojave iMac update to Monterey 12.1 WITHOUT a clean install but just by selecting Update in System Preferences.
To my complete amazement after skipping anything related to Catalina and Big Sur, in about 30 minutes I had a fully-functioning Monterey macOS. Files and apps are normal, appearance is new and in some cases better some cases worse than Mojave in my opinion, Time Machine continues to work including test restores of old files, all settings of importance to me were retained…very, very pleased. I know by not doing a clean install there are likely some dust bunnies/tumbleweeds floating around but if so they are not adversely affecting anything I do.
Two issues resulted: iTunes no longer exists of course so I had to find and get used to its new components. But all my home videos and music libraries were correctly placed in the new counterparts so just a slight learning curve for me. Also, I had to learn the new steps to back up my iPhones and iPads as I prefer to backup to the iMac rather than iCloud. The one loss I had was iPhotos. I had some (a lot actually) of photos in iPhoto that for whatever reason would not convert to Photos so while backed up I might not be able to retrieve them. Will work on this as time permits.
After two weeks, Monterey 12.1 is working very, very well for me. Of course, YMMV.