IME, Not interested at least prevents that single video from showing in my feed. It may reduce similar content from same or other channels--I see some reductions. Don't recommend channel always works for me as far as I noticed, except in search results which seems correct. I never expected it to not recommend similar channels, e.g. I filtered out Linus Tech Tips but watch similar content.
It probably helps that I only permit a handful of specific topics: physics, fun math, synthesizers (but not modular), tiny bit of music theory/training, StarCraft 2 (not SC1/BW), and recently the Nvidia/AMD GPU release saga.
It probably helps that I only permit a handful of specific topics: physics, fun math, synthesizers (but not modular), tiny bit of music theory/training, StarCraft 2 (not SC1/BW), and recently the Nvidia/AMD GPU release saga.