Carabiners are made from steel or aluminum and have historically been used in rock climbing and other forms of rope access. The use of carabiners has roots in the early days of slacklining. However, carabiners are not the ideal choice of connector for most higher tension slackline applications.
Firstly, carabiners are not designed for tri-loading. Tri loading occurs when three vector loads converge on the carabiner (like pictured below). For most carabiners, the load is intended to pass along the spine of the carabiner, while the gate remains comparatively unloaded. Because of how they are designed, the load stays along the spine of the carabiner when narrow objects such as rope, cord or other hard connectors are used. This is the case with most non-symmetrical carabiners like D carabiners and pear shaped carabiners. Heavily tri-loaded carabiners can lose more than half their breaking strength. When cross-loaded, they usually only hold around a third (it even says so on the carabiner). Perfectly oval carabiners handle tri-loading the best, but this does not mean that it is okay to use them in anything except very low tension, short slacklines or rodeolines.
When filling out your gear stash, we recommend only buying your connectors from trusted sources such as slackline shops and climbing stores. These are the people carrying the good stuff. Be weary of equipment in hardware stores and discount markets, you have no idea of the quality of the connector. Look for WLL and MBS ratings and understand the difference. Use carabiners in primitives and rodeolines only, where loads stay below 3-4 kN. Carabiners should not be used in any other high or cyclic load bearing application in slacklining.