Blending in a simple shader not working in Unity5 as it used to work in Unity4

I have a really minimalist shader, where all the content is this: Shader "foobar" { SubShader { Pass { Blend Zero One } } }

In Unity 4 this used to do what I wanted: it basically blocked everything behind it from being rendered. Well, in Unity 5 something really weird happens. It works, but only with about 80% "accuracy": depending on angles and distances some objects appear fully. So as I move the camera, things seemingly randomly appear and disappear.

What the heck is going on there? What has changed from Unity 4 to Unity 5 related to this? Why doesn't the shader work properly? What on earth causes this seemingly random behaviour?