For Epic Skeleton mode rigs, some twist bone controls might not work or work unexpectedly. This is mostly limited to location and scaling. Rotations work the same as rigify except for the first twist bone which is a special case.
For Free Form mode, rigs match the original rigify rig almost entirely. But there may still be minor differences.
IK Stretch might cause issues with child bone scaling in all 3 directions instead of just 2.
Yes and No
Rigify has been around for a long time and many of the rig samples (parts) are now deprecated. Uefy 2.0 is designed around the modern metarig as it is meant to be used in Blender 2.83 LTS and above.
In most cases a few adjustments to older metarigs options will make them compatible. But you must edit the metarig samples to update them as necessary.
You can add Sample types listed below:
In most cases you have to use the default human metarig as provided by blender. Or you must construct your own metarig but do so in the correct order from root bone onward.
Metarigs from other source may not meet requirments.
(See FAQ on which metarig samples are supported)
The process of rigging a mesh with Uefy Script requires applying the armature modifier to the mesh. Blender does not allow this if the mesh has shapekeys. You will need to delete the shapekeys to rig the mesh.
You can create new shapekeys after rigging is complete or you can try to copy/transfer shapekeys to the rigged mesh from a different object. However this is a complicated process that might require custom scripting. Refer to StackExchange and Unreal Forum for examples of how people have done this.
Since Uefy v2.2 there is a new tool to transfer shape keys that will work if the source and target mesh have the same vertices and pose.
The Shape Key transfer will only work on shape keys where the source and target mesh have same vertices and are deformed in the same pose.
For example you should be able to transfer shape key from the face because the face pose will seldom be changed but not from the hands or other parts of the body because we change the target mesh pose to match the unreal mannequin.
Match Pose will only work for animations retargeted on to characters rigged in Blender using Uefy.
For example you will not be able to export the original mannequin animations directly to Blender. You will need a mannequin rigged in Blender as shown in https://youtu.be/8Ad4v7x41RA . All animations retargeted on that Blender rigged mannequin will be usable.
Once your modified/new animation is back in the engine you can retarget it back the original mannequin if you need to.
This can happen in IK_Strech mode with some bones attached to the main skeleton. Those bone need to scale in 2 dimensions instead of 3. You can either manually try and add different constraints to limit scaling to 2 dimensions or just avoid IK_Strech if it becomes too complex.
You could try setting the default Copy Transform constraint to 0.0 on the affected deform bone.
Then add a Copy Location and Copy Rotation transform with the same parameters. This should fix most issues but the real fix is to come up with a constraint mechanism that only copies scale in the correct 2 dimensions.