Changes to the RenderViewportScale value cause problems that won't be acceptable in games, including:
* visible occlusion mesh
* duplicates of scene geometry blinking in and out of view
* scene geometry being offset by different amounts depending on the RenderViewportScale, sometimes off-screen entirely

Exact combinations of symptoms vary depending on RenderViewportScale value and Stereo Method, and whether you are using a built Player with Oculus SDK versus Play mode with Mock HMD.

UPDATE 12/15/2019: These problems also occur using Universal RP 7.1.6 with Unity 2019.3.0f3 and XR SDK on Rift, though the symptoms differ slightly from what was originally reported, and are a little different depending on whether HDR is used. For Stereo Instancing the occlusion mesh is visible and it moves as the RenderViewportScale value changes. For Multi Pass Stereo the whole viewport (not just the geometry) moves around in the HMD, though the scene continues to respond to head tracking.