Change my mind: train roundabouts are OK

I constantly hear people vilifying roundabouts like they are sent from the devil. And it seems that it usually boils down to "you can crash into yourself" (which is not relevant to me in 0.17 with the new temporary stops, I never manually drive my trains). I've heard conflicting opinions on loops and their effects on UPS and I'm really doubting the veracity of such claims. Since I don't know much about the topic, I would love to tap the hivemind.

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What are all the disadvantages of using roundabouts and loops? When you answer please be as specific and explanatory as you can - I promise I will read every word.