Well I would have to see the project but I can confirm that we have had no major issues using G3 no more or less than one would experience using any other custom based rig.
The G3 rig does have some variation from the stock Epic rig but easy enough to reconfigure before committing any animations.
What will not work is things like 2 bone IK or any physics dependent on an IK type solution. This is caused by the parent,parent linking of the bones normally used for twist instead of the expected parent, child link of the Epic rig.