About the optional dependencies (list borrowed from arch):
calf - limiter, compressor, exciter, bass enhancer, and others
lsp-plugins - equalizer
mda.lv2 - loudness
rubberband - pitch shifting

You will need to rebuild gst-plugins-bad after installing any of the optional
dependencies, pulseeffects will then pick them up and make them available,
otherwise, all the plugins are shown but none of them are useable.

If you run into issues, please try removing GStreamer's cache `rm -rf ~/.cache/gstreamer-1.0`
and have a look at the debug output with `G_MESSAGES_DEBUG=pulseeffects pulseeffects`

You can check what plugins your current gstreamer build offers with
`gst-inspect-1.0 | grep -i calf | grep -i limiter`

Additional info from the official FAQ over at https://github.com/wwmm/pulseeffects/wiki/FAQ

Question 13 - After turning on effects for an application my master volume is set to 100 %.

Answer: Take a look at the output of the command pulseaudio --dump-conf and
see if you have the option flat-volumes set to yes instead of no. For more
information take a look at #346: https://github.com/wwmm/pulseeffects/issues/346