I had an earlier Furuno unit for 14 year which didn't have a chart overlay. (One at the nav station and one at the helm). I had a chart plotter next to at both the helm and at the Nav Station.

I upgraded to the Furuno 3D with 12" displays about 4 years ago. It has overlay capability, split screens,. I still have the Northstar chartplotters near the Furuno displays.

I seldom use the radar in overlay mode. When I do, it's really for orientation and almost always briefly. There's just too much information on the chart that is distracting at studying the radar.

If I was on a HUGE boat with (say) a 24" to 40" it might be somewhat different. But in my experience, there is too much subtle information that is necessary to interpret to be distracted by the chart overlay.

Examples: some targets fad-in and out. Some double images (like a barge) are pretty subtle. Some daymarks and "floats" (on large fishing buoys) are "in-" and then "out".

When you add AIS targets, and you're pulling up info on the specific targets, etc. It just makes things more complicated.

That's just my experience.

Radar is a very neat tool, but its as much an art to get the most out of as a science. Kind of like reading and x-ray or a scan, if you've looked at those. Sometimes things are VERY obvious -- but often the most important information requires much more careful study and non-distracting display.