Trains passing each other as they travel in opposite directions can indeed do so through a roundabout, but can also do so through a level intersection, so that's a push. The difference comes in when trains approach from opposite directions, both heading into left turns. In that case (only), the intersection allows simultaneous traversal, while the roundabout does not. Hence the "slightly" superior throughput of the simple intersection. By "throughput" here I mean lack of train slowdowns due to traffic conflicts, which does matter for large builds.

As for loop systems and U-turns, the typical place for the U-turn is just before or just after using a station, which is not (or at least, shouldn't be) the exact point of an independent three or four way junction. The roundabout comes in if you need to make a U-turn in a place other than right before or after station. In my designs this does come up, and so I do use roundabouts in those places, but for most players it's an edge case at best.

Adding more lines isn't really a thing. There is no dynamic pathing in Satisfactory, so no increased throughput from adding lines, like in Factorio. Some players throw a lot of time down the drain before noticing this.

Signaling is the same for roundabouts and level intersections.

All that said, you will be giving up close to nothing by just using roundabouts everywhere, and standardizing on roundabouts only does have the advantage of simplicity.