https://docs.unity3d.com/Manual/Constraints.html

This system seems to be far better suited to perform a lot of task previously accomplished with other comments, while providing additional options for avatar creators

Allows limb copying without fixed joints (allowing avatars with multiple arms or legs)

Allows using world particles in situations that IKFollower doesn’t allow (Attach to non-IK-following bone) that previously required fixed joints

I believe these allow you to Animate holstering / unholstering weapons without needing to duplicate the mesh (IIRC, FinalIK supported this)

Aim/LookAt constraints can be used to replicate hydraulics pistons or other mechanical joints for robotic avatars (previously this could be achieved with dynamic bones any maybe FinalIK) These components likely can be used to achieve other effects.

These components hopefully won’t have the disadvantages of the systems they replace, such as;

IKFollower: only allowed on IK-following bones (As per documentation)

Rigid Bodies: Known to break in Combat enabled worlds (https://vrchat.canny.io/bug-reports/p/avatar-rigid-bodies-break-in-combat-enabled-worlds)

Rigid Bodies: Some configurations of joints can drift out of place over time or during teleports.