Being in something that's starting to look like a triad (three couples within a group of 3 people...) and also part of what I call an N (long distance relationship with a married woman), I suppose I could talk about either side of this conversation.
So far, the replies here say "I have nothing against triads," which I believe. At the same time, in the contentious/closed threads, people who write in talking about their personal circumstances (say, starting married and perhaps opening things up in small steps, or enjoying being with couples because they like the dynamic of joining something established) and get pounced on with questions that others seem to not get asked. Have you thought of THIS, have you considered THAT, and so on. Many of the questions imply that the member of the couple or the person joining a couple is a dumb-ass, or self-centered, or rude to the others in the relationship, or lacking in the wisdom that is this forum. Which may be true. Yeah, people opening up don't know what people who have lived this way for a while know. But the pouncing is what looks different.
But I don't want to make too big a deal of it. Someone just posted about the distinction between family-style and hub-and-spoke style poly, and the critiques that both sides get. Maybe it's just that more family-style attempts at triad life are showing up here, and people don't typically write in or seek advice when they're looking for a V? Or Z/N/longer chain? I don't know.
Whatever the case may be, I know that it's been really helpful to have people's perspectives from when their relationships have worked - no matter what kinds of relationships they have! I come here to celebrate success and learn from it, and I don't care if that's about triads or not.
Don't know if this response helps, but those are my thoughts on the issue, for now.