I voted yabu. Mainly because telling people they shouldn't do something is never going to work. I have seen blended families work.
But I do agree the odds aren't in their favour.
I do think it's a really bad idea to blend families when there's already back ground problems. Kids not happy or exs that cause trouble or a new partner not happy with how much is being paid to an ex.
Its sounds hypocritical. But when I was a single parent, I wouldn't date people with kids. I was also perfectly happy if someone didn't want to date me, because I had kids. I had always been clear, I wasn't having anymore. I had no interest in having anymore and I wanted to concentrate on my kids and getting them through the aftermath of a split. That bits takes a long time.
Just after the divorce was finalised, exh announced he was seeing someone. He introduced fast. I kept my nose out because it was obvious he had been seeing her a while. 12 weeks later he moved in with her and her 2 kids. And my kids were expected to refer to these kids are their brothers. It lasted 4 months, with exh announcing it just wasn't for him 
Dd, who was 12 by this point. Absolutely refused to meet the next woman. And hasn't really stayed at his house since. Exh has been engaged twice since then. Ds, had also recently stopped going as much.
My, now dp, does live with us. We took it slow, introduced him slowly. He has no kids. Though he does have an adult step son from his marriage. They are still close.
This has worked for us. One of my favourite things about dp is that when one of the kids is moody, he can calmly tell them it's not ok to be rude. But then let it go and they are laughing and joking with him 5 mins later. He tries not to take their dad's place, but (given their dad has basically checked out) tries to be someone they can talk to that's not me. He is happy for them to take the lead on what type of relationship they have with him.
But I can Absolutely say that if it got any whiff that it wasn't working for one of the kids, we would split. I am the higher earner and we live in my house. I actually enjoy being single. I just wanted to be with dp more than I liked being single.
I can (financially & emotionally) go back to being single. To the above poster who thinks their mum was never happy, that doesn't mean a relationship would have made her happy.
I won't ever get married again and do thing very carefully and pragmatically about these decisions. I had some pity thrown my way yesterday on a thread because I clearly think about relationships with my head too much and don't follow my heart enough. 
So from my own situation I would say blended families hasn't worked out for my kids. My daughter, is already very against ever dating someone with kids. It's really left a mark on her.