Unlike @Shamino I actually use Mail.app a lot, basically almost exclusively (apart from rare web interface use or mutt sessions via remote ssh). I can only say that although I was very worried about the Mail.app bugs initially, and read and double-checked everything I could on the matter before hesitantly upgrading to Catalina, it ended up working without a hitch.
And to this day, my Catalina Mail.app has been performing exactly as it had under Mojave. Mostly well, with two or three annoyances (one I just recently noted here on TidBITS Talk). I have to this day experienced zero email loss or had to restore anything from backup and that’s the case for all my accounts: iCloud, Gmail for work, and my private IMAP server for personal stuff.
Not trying to generalize or say these issues aren’t real (I have no doubt they are), just trying to point out they are not necessary universal and that for some of us there has been a path forward with Catalina that appears to have worked quite well. Thank goodness, since you really have no choice when you get a new Mac.
[I realize I probably should never have written this and that I just jinxed myself. Ugh.] ;)