It's... it's meh.
I think the only reason people really ship this is because Kirumi kills Ryoma, and while the way Ryoma just sort of accepts his death and Kirumi fulfills his wish and kills him is kind of sweet, they spend little to no time together and their personalities aren't all that compatible. Ryoma needs somebody who can help him through his depression or at least be a ray of positivity for him to get through life, and Kirumi, while not necessarily cold, is very closed-off and doesn't express her emotions very often.
One thing they do have going for them, though, is that they're by far the two most mature characters in the game. While everyone else is dicking around being teenagers, Kirumi is hyperfocused on serving others with total unseflishness and Ryoma is coping with life in prison. There are some characters who have suffered more like Maki or are more intelligent like Kokichi, but when you get down to it, they're still kids in many ways. Ryoma was forced to grow up quickly due to his whole murdered-an-entire-mafia situation, and Kirumi sees it as her duty to behave as maturely as possible for the good of others. Their relationship wouldn't be the most colorful or compelling, but it would be interesting simply because they're the only two adult-minded characters in their game. Thinking about it, I'd love to see more of them together and am kind of pissed now that their only real interaction is when she's about to kill him.
I kind of see the merit, but honestly, I think they're more compelling as friends. They don't have too much chemistry, but a relationship between them would definitely be intriguing, so I can't rank it too low.
6/10 romantically, 8/10 platonically.