That's where subjectivity comes in. A song doesn't need to be catchy and "head-bobbing" in order to sound well. And the song doesn't need to build up towards something epic, the minimal sound is what's so epic of it: they've created a more than calming song with just one chord progression and synth, the modulation of that synth is the subtle detail that keeps the song from just being an endless loop, but the overall simplicity of it is to marvel at. Boards of Canada prove themselves to be skilful by epitomising the capacity of minimalism to convey the same strong emotions that the ability of complexity has as well. They could have easily created the same atmosphere using their entire arsenal of musical knowledge, but the fact that they did so with limiting that knowledge, highlights their music is both powerful and amazing.
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7/10
Here's another great example of minimal sound. One chord progression, one synth, one beat pattern but diversified with the screams in the background and a speech by Alan Watts on "what is I?"