Development

Unity has its own weard texture lighting system and Unreal needs two different rigging depending on if the model is player controlled or NPC so simple answer is no

However you can (some already have) use Blender with 3rd party plug in... The issue then becomes the fact that Fbx isn't a real standard so development needs to be constantly updated and most people will give up after making a app that doesn't really get used a lot

there is no 'Mixamo development team' Adobe grabbed Mixamo as part of Fuse in order to get their hands on the staff who now are the Stagger team

what are you trying to develop? if the answer is computer AI then forget it but anything else is ok as long as you embed the assets