I've used RemoteUtilities for a couple years to successfully access a computer and see all 4 monitors remotely.
Since upgrading to version 126.96.36.199 on both the host and viewer, only the first and second monitors can be seen. There is appropriate black space for these monitors on the "Entire desktop" view. They will not show when attempting to view individually.
To add to the mystery, the image on the Remote Utilities - Viewer shows images for the first 3 monitors.
The host has been upgraded to Windows 10. We only now installed the new RU version, so perhaps the Windows 10 upgrade is a contributing factor.
The "Second screen black" thread is similar to this, but since that was a new install, I've started a new thread.
Are there any downsides to this legacy capture mode?
Not at all. As the name implies, this option was added for compatibility with other Windows capturing mechanisms. This is a simplified explanation though, so to say. Most users only care about the end result, which in this case is triggered by a single check box. All that is needed. :)