I'm going to give you my basic opinion of Shumon Yuu straight up and without embellishment... he is a genius.
I generally am reluctant to call any writer a genius. I have read thousands of stories - if I include both books and VNs - and I can only name a bare dozen or so authors/writers I can honestly and unequivocally name as geniuses. Of course, this is a subjective viewpoint... but it is backed up by significant experience, lol.
Shumon Yuu is that rarest of the rare in VN writers/directors... a true artist. Most decent VN writers have a flair for some aspect of their work, whether it is characterization of a certain type or amusing narrative... but Shumon Yuu goes past that, turning entire VNs into works of art as deep and expressive as any classical piece.
I'm not talking about him pleasing me on every aspect... several of his VNs lie outside my tastes to one degree or another... but it really doesn't matter when I'm reading one of his works. It doesn't matter that I don't like a certain character or a certain plot element. When the VN is complete for the first time, I always feel like something about what I just read ripped deep into me and tore out pieces of my being I hadn't yet known existed, bringing them into the light for me to see.
I have read three VNs he wrote and two he helped plan/design. The three he wrote are undeniably kamige, with a wide appeal and a unique approach to storytelling (which differs radically with each one) and the two he helped with are first-class VNs. Chrono-belt, which is the crossover fandisc for Ayakashibito and Bullet Butlers, is such a work of genius at capturing the best of the spirit of those two games that I still get the urge to play it independently at times. Kikan Bakumatsu Ibun Last Cavalier I named VN of the Year 2015... though that isn't really saying much, considering how bad a year that was (it did have a fascinating take on the Bakumatsu era though). Tenshi no Hane o Fumanaide was my second game by this writer, and I have played it three times now... each time rediscovering what made me fall in love the first time. Kitto, Sumiwataru Asairo yori mo is a work of nakige/utsuge genius that still births new fans even today.
So why are Shumon Yuu's works primarily known only to people that are a part of the 'in' crowd of veteran untranslated VN readers in the West? It is probably because his works don't fit precisely into any of the existing/accepted genres, even if they sometimes use elements from them. You pretty much have to be an omnivorous VN reader to run across him, because it is difficult to impossible to fit any of his VNs into an archetypical aesthetic. Another reason is that he isn't very productive. In the past seventeen years, he has been involved with the production of precisely eight VNs... and he only wrote six of them. He is also an LN writer, apparently, but he can't really be said to prolific there, either. So... he tends to fall behind writers who produce something every year and jump at every chance to advertise their own greatness, lol.
He doesn't get recommended as often or as fiercely as Masada or Higashide, nor does he have the immediate impact of Akatsuki Works' writers. In fact, even I tend to forget about him (though not his VNs) for years at a time... until I read something he was involved with and begin dancing with glee once again. His works I never forget, but I frequently forget to follow him, hahaha....
Also, he is a pretty subtle writer, so most people won't pick up on everything he is trying to express in his games on the first playthrough... one of those rare VN writers who gets better as you chew him, lol.