Yeah, ‘heavy’ is a completely subjective term in this context.
Muse’s staple arpeggiated synths from the past, weaved in at the right moment, managed to enhance their dirty three-piece riffing sound and beefed things up a bit by providing contrast with the gritty guitar and bass sound. Their current stacking of layers doesn’t manage to recreate that same feeling, because it gets too muffled and it just sort of kneads it all together, taking away from the ‘heaviness’ rather than emphasizing it through contrasting sounds.
It’s like making your ‘freshest salad yet’ by adding pomegranate, feta and cucumber and then putting it in a blender. Doesn’t work that way.