In our entire history of using limited attempts, there was only one boss where I'd say they actually worked out fairly well: Heroic Lich King. The main reason for this was that you couldn't access Heroic Lich King at all unless you defeated Putricide, Sindragosa, and Lana'thel on Heroic difficulty as well. And during the first few weeks of Heroic progression, that was beyond the means of essentially all the alt raids out there. After all, the point of running split main/alt raids is to funnel gear to the mains to maximize their effectiveness for Heroic progression, right? So by definition the alts are less geared than the mains, and if the mains can just barely, barely beat something, the alt group is going to be in for a rough time.
As a result, I don't believe we ever saw a single true alt Heroic Lich King progression raid. Similarly, I don't think we're going to see alt raids on Ra-den. The number 30 is also large enough that while one accidental pull or disconnect is of course still going to sting, it isn't crippling.