A good roux can deliver a creamy bechamel or a deliciously dark and deep gumbo. To make an effective roux, you need to be mindful of how much butter and flour you're mixing together. You want enough for the flour to hold the butter, but not too much that it separates.
Knowledge and inputs are the same.
Take in only enough to be effective. Take in too much and you won't be able to keep together.
Cheers to moderation.