We knew going into Rogue One that it was a one-off deal. Other than Darth Vader, CGI Tarkin, and maybe a few others, any character would be introduced and then ultimately die because we already know that they're not around in Episode IV.
But we all knew that, including the guys making the film. I think they would have done better with fewer characters, allowing us more backstory and/or character development for each. The more invested we are in the characters, the more impact it has when they ultimately meet their fate. When Whatsername and Whatsisname are sitting on the beach at the end waiting to die, I thought it was well-done, except I wished I cared more about the characters. I've seen the movie twice, and honestly right now I can't think of either of their names. I found myself watching the movie and thinking about how well they've connected things to the events of Episode IV more than I did about the characters.
Rule #1 in storytelling: You have to at least care about the characters. Since I didn't really care about them, there was no impact, thus the movie failed. That's a gross oversimplification, but that's what it comes down to for me.