There are no open security bugs against TDE that I'm aware of—if there were, I'd expect them to be fixed in the next release. In my experience, the development team, while not huge, is active and competent.
I've been using TDE since a little while after Gentoo sunsetted KDE3, and I've had no issues. Just make sure your X server is secure—-nolisten
and all that stuff—and don't try to use Konqueror as a web browser (it remains an excellent file manager), and you should be fine.
Wayland is "more secure" than X in that it makes less LAN contact by default and tries to sandbox programs from one another to an extent, just in case some future browser exploit that can copy random swathes of your screen tries to screenshot your password manager or something. There are no active exploits against a correctly-configured X server at this time that will magically vanish if you switch to Wayland, as far as I'm aware—it's more future-proofing stuff.