Some fonts might be unavailable for usage in Konsole, although they are available in other applications. That doesn't mean there is a bug in Konsole. Konsole requires monospaced fonts to provide the best visual result, so it asks Qt™ to only list monospaced fonts.

Starting with version 16.08 (August 2016), Konsole can be configured to allow selecting any font with the caveat that the display may not be correct.

Since KDE4 all the tabs use the same process ID. This has the side-effect that if one tab's process has issues, all the other tabs may experience issues as well.

This is most noticeable when a command that connects to an external device or system (ssh, nfs) has issues.

Konsole treats arguments after the -e option as one command and runs it directly, instead of parsing it and possibly dividing it into sub-commands for execution. This is different from xterm.

Konsole doesn't provide convenience for running login shell, because developers don't like the idea of running login shell in a terminal emulator.

Of course, users still can run login shell in Konsole if they really need to. Edit the profile in use and modify its command to the form of starting a login shell explicitly, such as "bash -l" and "zsh -l".

The --new-tab option sometimes behaves strangely. It may create new window, or it may create new tab in another existing Konsole window instead of the current Konsole window.

Those behaviors feel strange, but they are not necessarily bugs. The --new-tab option tries to reuse existing Konsole windows, but not all Konsole windows are reusable. All Konsole windows opened through KRunner are reusable, while most Konsole windows opened from command line are not.