Some of the issues that one may experience is related to the new security requirements where software such as plugins require to be from identified developers. I personally found a major issue with this requirement relating to Rapidweaver add-ons that had not been signed. Basically it prevented me from using any not signed. I now have to wait for the few add-ons to be updated before I can use them again on my Catalina installation. Fortunately I only updated the MacBook and not the iMac where I perform most of my work. I found the same issue with one of my plug-ins for QuarkXpress that stopped working.
Another major change is that 32 bit apps are no longer supported so it is best to check if any of your important apps are 32 bit before updating. You can do this by going to About this Mac/System Report/Applications and then sort the 32/64 bit column. This will list all the 32 bit apps. If any of them are critical, do not update.
In my case, I have kept my main machine on Mojave and created a Mojave VM where I installed the critical 32 bit apps for the short term.
There have been a number of articles on all the new features of Catalina so I will not repeat what is already readily available but the new OS looks great and has provided many improvements.
Check out https://macmost.com where Gary has released several videos on Catalina.
Other sources include https://appleinsider.com and https://9to5mac.com as well as many others.
Of course Apple has information about all the new features as well. https://www.apple.com/au/macos/catalina/
Let me know by email or completing a service request form if you would like some more specific links or help with Catalina.
Information about the new features of the new operating systems for the different platforms is available on a variety of sources such as AppleInsider, Catalina Preview , Apple Developer - Beta 2 Release Notes as well as a host of other locations such as TechRadar that reported on the WWDC 2019 event.
One of the many significant changes are with the MacOS which will lose support for 32 bit apps and the iPadOS will have much better functionality with using storage devices for files. Something that has been a long time coming but most probably held back to avoid cannibalising the Mac sales.