For upgrade, the biggest breaking change is the DOM structure. So depending on the styling of the project you might need to do so work https://docs.mendix.com/refguide/moving-from-7-to-8#9-2-dom-and-atlas-ui-issues
From the widget API, some function have been removed (They where deprecations in Mx7). This might break some of you custom made widgets. More and more app store widgets have been update. (There was a tool to help you out, but it not recently updated :( https://github.com/mendix/mx-check-deprecations ) Maybe one knows an alternative.
A common issue https://forum.mendix.com/link/questions/94688
If you are depending on “private” Mendix API (undocumented) you can face some problems as many Core has changed much.
When you are stuck you can reach out to me via the Community Slack Channel.