I was just thinking about this yesterday. One of the things I found most moving about Boyhood was that, in addition to seeing the actors age, we got to see the world change around them. Outdated technology or passing fads weren't inserted as cheap references, but because they were actual aspects of living in middle America at the time the movie was being filmed. Seeing a Gameboy or a defunct chain restaurant in Boyhood hit me in a way that a movie shot today wouldn't.
Merrily is a period piece. If you're already going to have to build sets and source props to recreate 1950s to 1970s New York, then filming in real time doesn't seem like it would have as much as an impact as it did in Boyhood. Unless they're doing some radical reinterpretation of the material where it's updated to take place in the 2010s to 2030s.
I loved Boyhood, so the idea of filming over time is not an issue with me per se; however, given years they are taking for this film, so much can go wrong. I continue to think this is a really dumb idea; probably angry because the probability that I will ever see this gets less likely without any delays to make things worse. If it turns out to be a masterpiece, at least it will live forward.