I really believe magnesium has helped my young horse (I've used several different supplements with magnesium in them). I even let him run out for a couple of weeks and his behavior started getting questionable again so I put him back on it.
He's not a bad horse, but he's only 5 and sorta spooky, especially riding alone. So yeah, I am a believer.
Maybe some day I can wean him off of it, but not yet.
I would guess Smart Calm Ultra has a goodly dose of magnesium in it, most of them do. I only tried one supplement that had a very low magnesium dose in it (VitaCalm I believe) and I don't think it did much for him. I didn't try it for very long, just a 4oz dose before a couple of rides ride (per package instructions) and I don't think it helped. But I am giving magnesium daily and I do think it helps. And lunging when he's had time off. And not overfeeding the alfalfa (about 1/2 his diet is alfalfa).
There's a lot of stupid out there!