Are there any downsides to K-9, unintended consequences, etc.?
The program works as you'd expect an email app to work. It looks similar to the default app, and includes extra features like email signature, changing from a dark theme to a light theme and vica versa.
There are a few things I don't like. It's so advanced, that it's cumbersome to set up. I have an email account from my university, I set it up as IMAP, but K9 showed and wanted to pull (or push, your choice) every single folder (including Junk, Sent, unimportant stuff). You have to manually go through and configure each folder how you want it to update and such.
Once that's all said and done, it works well. I think it MAY be bigger in RAM than the default app but I might be wrong, it could be the same.
One thing I don't like: in the default app, if you open a notification saying you've got email, it will take you straight to the email, but in K9, if you do this, only SOMETIMES does it take you to the email. If you were viewing another email account, it will instead take you to that account, and you have to go back, select the account you want... It's annoying, but it adds at most 3 more seconds of effort.
There are probably many more features I haven't explored.
Oh, and it works perfectly fine IMAPing into Yahoo mail, of course...