It's just my way of preventing a disaster. And I can make awesome buerre blanc (French butter wine sauce) but in the early days of cooking I destroyed a roux or two with lumps and not stirring quick enough. And back then I didn't have electrical gadgets to save me. So the whisk became my friend. My gf who was a saucier was an insane stirrer. Her arms were almost robotic with how fast she could stir. Her sauces were amazing. And the texture was always divine.

as for the separation? Nah that would just make it taste floury but not be lumpy. Sometimes not enough butter and overcooking the flour can cause undissolvable lumps (yeah I totally made up that word!) because they cook into miniature dumplings in the butter almost.