When comparing Krunner vs Ulauncher, the Slant community recommends Ulauncher for most people. In the question“What are the best app launchers for UNIX-like systems?” Ulauncher is ranked 3rd while Krunner is ranked 8th. The most important reason people chose Ulauncher is:
Ulauncher comes with very reasonable defaults, looks good and just works.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Opens folders/files also
Just by typing name of file or folder, it will give recommendations

Pro Extensible with plugins
Krunner can be extended with the use of "runners" which are essentially plugins for the app launcher to allow for extra functions.

Pro Fast
As it is written in Qt QtQuick using QML.
Pro Lots of handy features
Such as a fully featured calculator, file opening, browser functions, currency conversion, date/time functions, etc.
Pro Great integration with Plasma, developed by KDE
Pro Can be used to open websites
By just typing an URL into the text box users can have the site open in their default launcher.
Pro I'm able to find open tabs from browsers with KRunner which is very nice
Even supports edge browser.
Pro Simple
Ulauncher comes with very reasonable defaults, looks good and just works.
Pro Fast
Pro Allows custom shortcuts
Pro Themeable
ULauncher supports themes and comes with a good supply on install but more can also be made.
Pro Supports fuzzy search
You can enter app name with a typo and Ulauncher will still be able to find it.
Pro Supports extensions in Python
With Ulauncher you can easily add and update third party extensions. From kill process to windows switching, many of them work very well. The extension API is documented and if you know Python you can create and share your own extensions.
Pro Actively maintained
Unlike Albert or others, it is actually being maintained.
Pro Good extension ecosystem
Pro Good look
Ulauncher looks modern & can be customized with many themes.
Pro Direct plugin install from GitHub
Can fork, customize, and install instantly.
Pro Extensions are really easy to write by yourself
Pro Quick search
Pro Able to launch Kdialog, Zenity and Bash scripts
Extensions are easily created. Python script is all open to see. Not compiled like some.
Cons
Con There are few plugins/"runners", and some do not work as they are old
Con Extensions store is confusing, lots of redundant extensions.
There are dozens of extensions, which is good, but as an average user it is hard to find what you are looking for. It is unclear to me how updating plugins work.
Con Needs more posibilities for extensions
Currently, extensions can run only when you type their alias and <space>. There is no way to see results from multiple extensions at once. The output is also limited just to rows.
Con Does not easily fit into a customized desktop without full GTK3 CSD support
