When you open too many files for the screen, KATE tab bar no longer lets you scroll to more tabs like one KATE tab bar did in KDE4 (multiple lines of tabs would also work.) Those files are just inaccessible unless you go to some menu, not a user-friendly way to edit a large session.

I put it as 'wishlist,' but personally, this is so critical I'll likely downgrade to KDE4, TDE, or KDE1, from P5, because I often edit many large sessions (of course, earlier KATE had certain problems, but some were more tolerable.)