The reviews I've come across that are not reflected below are about evenly split. This result may be in line with or even a little better than what everyone involved expected. The "bold" and "brave" "little indie" movie is finding its audience, albeit one that was always expected to be very small. Its target market is a tiny subset of today's movie goers. They must not only appreciate European art film, but in particular the very slow, moody, doomed lovers kind. Among movie critics, only those who recognized the similarity to films like Antonioni's La Notte, or Who's Afraid of Virginia Woolf, fully appreciated the subtleties and nuances of what they were seeing.
It is no surprise that there are critics who hate the movie -- and apparently even the very concept of this type of cinema. But even among those who didn't like it, about half still liked certain aspects of the film and respected the effort.
It's early and most movie scores edge down after the greater mass of critics weigh in. But for a film that has very, very limited appeal, By the Sea has already won support from movie nerd sites that in the past were more critical of both Unbroken and In the Land of Blood and Honey.
After the dust settles, the net result especially among the movie nerds is probably greater respect for her knowledge of cinema, her talent and passion for directing, and her versatility behind the camera. When they sit down for one of her future directing efforts, they can't be sure of the type and tone she will be presenting especially if she also worked on the screenplay.