GLMIxer is a great free and open-source VJ software to get started VJing. Like any other software it has its own limitations. Let's see some aspects to be taken into consideration:
Video mapping. The name GLMixer comes from Graphic Live Mixer, so mixing video footage is its main function. If you are interested in video mapping, you may prefer an all-in-one proprietary VJ software or an open-source alternative such as MapMap.
BPM sync. It is an essential feature when it comes to music events, unfortunately at this time you can't import audio into GLMixer or receive external audio signals. For that reason, you won't be able to generate audio reactive visuals inside GLMixer for now. Alternatively you can control some parameters via OSC.
Stability. The application is stable enough, however, if you intend to use third-party plug-ins, be aware that the possibilities for your app to crash are high, especially if you are not a macOS or Linux user.
Apart from the above-mentioned points, you still have access to a bunch of cool features for VJing. Watch the video below to learn more. In order to download the VJ loops used in this video tutorial, check out the Mech VJ Loops Pack.