There are a few reasons you might not want to use a rigify style rig.
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Blender scene units are not correctly set to avoid issues, so you have to run a script before exporting every animation to create a new rig with the fixed settings (instead of changing the scene unit settings once and be done with it). This will also cause problems with custom rigs, for example if you make a hair rig (or whatever) in the same scene it won’t work properly if the root bone is removed.
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Unapplied 0.01 scale on the mesh because of the above workaround, which could make newbies think that having unapplied transforms on meshes isn’t a bad thing, which could screw them over later.
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When running the script the armature is replaced by the “fixed” armature, so when you want to animate again you’ll have to delete the “fixed” armature (and the empty) and add the normal armature back in the armature modifier because it is removed after deleting the fixed rig (after exporting an animation the mesh will stop responding to the rig every time).
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Because of having to run a script before exporting every animation you can’t run other scripts like normal animation batch exporting scripts for exporting every action in the file to a separate .fbx file. Or at least they become way more complicated.
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The bones do not have the right names, so if you want to retarget you’ll have to select the right mapping in the retarget manager which is a bit strange for a custom rig, it should just autodetect the bone names (by using the correct names in the rig). We saw this in action in the stream.
Rigify was never meant as a rig for games and anything to make it work like that is more or less a hack (as you can see in the script). It can work as showed in the stream but it takes more time to set up than should be necessary.
I prefer the UE4Tools rig (http://www.lluisgarcia.es/ue-tools-addon/) because:
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It forces you to use correct scene setup (0.01 metric) because the rig is that size.
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No unapplied transformations.
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No script required after generating a rig, you can use it for multiple animations exporting the same rig (control rig with deform only ticked) instead of having to create a new one.
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Ability to run batch export scripts or other crazy scripts (see UE4 animation retargeted(?) to Blender control rig - YouTube for retargeting UE4/marketplace animations to the IK control rig).
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Correct names for retargeting by default so you don’t have to waste time doing that.
It’s also on github (GitHub - lluisgarcia/UE4-Tools: Blender addon for improve the workflow between blender and Unreal engi) and uses templates for the rig so it could easily be expandable by the community in the future to support other kinds of rigs like four legged rigs and so on. I think it’s a better alternative all things considered.