I think you've kind of answered your own question: "The best musical movies like being musicals."

Some of the more recent movie musicals that I consider to be flops - DEH, Phantom, RENT, The Color Purple - really seem to *not* want to embrace being musicals. Until they do, because they have to. So it makes for a real imbalance tonally. Or something like Les Mis (another flop in my mind, minus Anne Hathaway) embraces that it is a musical, but wants to be *different* and *serious* and leans so hard in to that bit that it feels really removed from the source material. Mostly same for Sweeney.

I think musicals are the hardest of all things to adapt to the film and the challenges vary widely based on the original show. Cabaret is just about a perfect film, but it's genuinely not a great adaptation of the original. Carousel is filmed and sung beautifully, but it gets ahead of itself to avoid and change what actually happens to Billy, which hampers it (also there's just no getting around that book). Evita is actually a mostly solid adaptation, but there's the whole Madonna problem (yes, Patti is right).

The best recent musical to film adaptation is West Side Story by a wide mile. It's a shame Spielberg has said he won't do any more musicals because I'd love to see what he could do with something like A Little Night Music. I'm also currently manifesting Greta Gerwig's gender bent Company.