On Saturday, via my Mac Mini, after Erasing and Formatting my Samsung 860 EVO 500 Gig SSD in an external Case, I did a clean, "virgin" installation of oS 10.15 onto that SSD, and then migrated/copied needed files, folders, settings, etc. from my (Mojave) internal SSD on my Mac Mini. That process went fine.
Restarted the Mini from that external SSD, and had no issues. Of course my 32 bits apps that were migrated were "greyed out", ie, disabled, which I expected. Tested Thunderbird, LibreOffice, Google Chrome. Firefox, Quicken 2017, VueScan, VLC, 1Password, EasyFind, AppCleaner, and the "new" iTunes, and all worked well. And was able to print a Writer Document, via LibreOffice, to my Canon printer.
Catalina seems fine, but given that a couple of my critical apps have not been upgraded yet (SuperDuper!, TechTool Pro, and Onyx), I am not doing any more testing. Also, since Catalina has some bugs (typical with a new Mac OS release), I am going to wait until at least V10.15.2 of Catalina is released before upgrading permanently. There is really nothing "earth shattering" in Catalina that I need, and based in prior years, TechTool Pro typically is not compatible (with an update) until either the .1 version has been released (was the case for Sierra and High Sierra), or the .2 version was released (was the case for Mojave). And Mojave is working just fine.