Would clouds, water and foliage likely to be a problem with a 1/4 second delay?
The GFX has EFCS, so why run the risk of these issues by using the electronic shutter?
Clouds: almost certainly not.
Foliage on a windy day: possibly, but you'd have to look closely to see the distortions.
Water: specular highlights could look strange.
I'm guessing that this is not gonna be a problem for most landscape photographers. I've been using a Betterlight Super 6K for many years, which takes 30 seconds to several minutes to make a scan, and don't have to spend too much time retouching except for some subjects.
In answer to your last question, the only time I think you'd vastly prefer all-electronic shutter over EFCS is when acoustic noise is critical, or when doing time lapse work and making hundreds of thousands of exposures. I think this camera's all-electronic shutter will give anyone who tries to use it with fluorescent or line-frequency LED lighting fits. But that's true of the Sony a7x silent shutter, too.
Jim