Most teams plan an EF6 to EF Core migration as "swap the package, fix some namespaces, ship it." For straightforward CRUD codebases that's mostly true. For anything with real query complexity, you will hit a handful of translation differences that the upgrade tooling does not warn you about. The queries compile fine, the unit tests pass against an in-memory provider, and then production explodes the first time someone hits the page that runs the report.
These are the five that bit us hardest on real migrations. None of them are obvious from reading the official "what's new" page, and a couple of them are silent regressions in behavior rather than visible errors.
1. Client-side evaluation does not silently rescue you anymore
EF Core 1.x and 2.x had a feature called automatic client evaluation. If part of your LINQ query could not be translated to SQL, EF Core would happily run that part in memory after fetching the rest of the data from the database. It logged a warning, but the query worked.
EF Core 3.0 changed the default to throw instead of silently falling back. Code that ran fine for two years suddenly throws InvalidOperationException: The LINQ expression could not be translated.
