It's safe to use
You need to be careful while installing software. Pay attention to what's being installed and to what's being pulled in by way of dependencies. For example, you may have installed
dolphin, the file manager. That would pull in
libkf5baloowidgets5 among other things.
gwenview, an image viewer, would pull in
If you're interested in chasing things down, run
apt rdepends libkf5baloo5 and
apt rdepends libkf5baloowidgets5 and examine the output for software packages you may have on your system.
Under normal circumstances in Kubuntu 18.04, which includes
baloo by default, use of resources is minimal after the initial indexing process. RAM and CPU usage are not normally a concern. But there are reports of
baloo running amok even on pure Kubuntu! I use it without problems for indexing both filenames and file contents.