I didn't see Ronstadt in PIRATES (except in the film, where she's fine in a role that doesn't really require much characterization), but on the cast album that was made after the Broadway transfer, it's pretty clear that Ronstadt is getting some electronic assistance on some of the high notes. It doesn't surprise me a bit that she had even more trouble with BOHEME, even though she wasn't doing eight performances a week.
Was she able to sustain a performance in PIRATES doing eight performances a week or did she have an alternate there? And was she able to hit all the notes as written or was any of the material taken down to allow her to actually do it live?
I really appreciate that Ronstadt, at the height of her career, wanted to try something different and chose two of the most difficult disciplines the performing arts have to offer: opera and Broadway. If she didn't enjoy either enough to ever return, I suppose that's fair enough. Not every discipline is for every person.