Everything wrong with blender rigging:

No unlimited bone layers

Bone layers are unnamed, needs supplementary addons or custom code to be considered animator friendly

Bone naming scheme is underutilized

Blender makes no indication of importance of the "." character in bones

Spline IK has no roll support (there are workarounds that are unstable and extremely complicated)

Limit distance constraint distance can't take in to account scale of bones it is interacting with (workaround = excessive drivers,
does not support non uniform scaling)

There is no concept of IK/FK switching, has to be implemented trough custom Python code

Drivers are associated trough a python reference string that rarely behaves as expected when changing the things it is bound to

THERE IS NO BONE EYE DROPPER, GOD HAS ABBANDONED US

Selecting a bone in complex rigs has horrible performance

There is no shear constraint or transform property

Object visibility can not be controlled via a driver for performance purposes, making proxy objects extremely limited

Object shape keys are resolved per mesh instead of per object, making importing multiple instances of the same rig/character/asset impossible

Shapekeys are resolved before the modifier stack, making weight modifiers useless for masking a shape key

There is no modifier equivalent of a shape key (transfer vertex positions), making all the problems above unavoidable

Custom bone properties can not be displayed in multiple bones, making for awkward or/and asymmetrical rigs

There is no Spline IK with a BBone chain as a target

Transformation inheritance is awkward and requires many drivers

There is no way to indicate how IK chain should be resolved on a bone with constraints, forcing duplication of chains what would
usually not be an issue but is a large problem as there is no unlimited layers and introduces lag every time a bone is selected.

There is no relative driver references (eg: current_object.parent["custom property"]), making things such as controlling a material trough
an armature impossible for multiple instances