I know it's supposed to be one problem per report, but I can't tell which of these things are the same bug and which aren't. It affects both Edit and Pose mode (although some of these properties only exist in Edit mode)
Broken: 2.80, 81b68f7279cd
The following properties don't seem to be considered by X-Mirror, at all. I split these into further two categories:
Properties that don't get mirrored at all, arguably not a bug, but I think it is:
(̶E̶d̶i̶t̶)̶B̶o̶n̶e̶.̶e̶n̶v̶e̶l̶o̶p̶e̶_̶d̶i̶s̶t̶a̶n̶c̶e (Edit)Bone.envelope_weight (Edit)Bone.use_envelope_multiply (Edit)Bone.bbone_segments (Edit)Bone.use_deform EditBone.lock EditBone.connected * (Edit)Bone.use_local_location (Edit)Bone.use_endroll_as_inroll (Edit)Bone.use_inherit_scale (Edit)Bone.use_inherit_rotation
*: Even if you argue that the Connected checkbox itself shouldn't be mirrored, the checkbox has the potential to change the bone's transforms, and these transform changes don't get mirrored until the bone is interacted, and the interaction is finished(not cancelled)
Properties that don't get mirrored at all, definitely a bug:
Edit/PoseBone.bbone_curveiny Edit/PoseBone.bbone_curveouty Edit/PoseBone.bbone_scaleinx Edit/PoseBone.bbone_scaleiny Edit/PoseBone.bbone_scaleoutx Edit/PoseBone.bbone_scaleouty Edit/PoseBone.bbone_easein Edit/PoseBone.bbone_easeout
The following properties work, but only after interacting with the bones in the 3D viewport, even if the interaction is cancelled (I'm guessing in these cases the mirroring worked, but a viewport update call is missing):
EditBone.bbone_curveinx EditBone.bbone_curveoutx EditBone.bbone_rollin EditBone.bbone_rollout E̶d̶i̶t̶B̶o̶n̶e̶.̶h̶e̶a̶d̶_̶r̶a̶d̶i̶u̶s
The following properties work, but only after interacting with the bones in the 3D viewport, and the interaction must NOT be cancelled, in order for the mirroring to take effect:
EditBone.tail_radius E̶d̶i̶t̶B̶o̶n̶e̶.̶h̶e̶a̶d̶[̶1̶]̶ ̶*̶2 E̶d̶i̶t̶B̶o̶n̶e̶.̶h̶e̶a̶d̶[̶2̶]̶ ̶*̶2 E̶d̶i̶t̶B̶o̶n̶e̶.̶t̶a̶i̶l̶[̶1̶]̶ ̶*̶2 E̶d̶i̶t̶B̶o̶n̶e̶.̶t̶a̶i̶l̶[̶2̶]̶ ̶*̶2 EditBone.bbone_z *3 EditBone.bbone_x *3 (Edit)Bone.envelope_distance
*2: These are mirrored fine when changed in the 3D Viewport, but not when changed in the UI.
*3: These can currently only be changed individually via the python console. They can be changed uniformally with Ctrl+Alt+S, but that works as intended.