I wouldn't worry about fire: it seems to be quite modern so most likely has quite some fire equipment and concrete. Common parts from the pictures don't seem to have much flammable stuff. Last time I stayed in London was in a hotel assembled from multiple old 3-4 floors buildings and multiple old wooden staircases, like to go to your room you had to go to floor 2 then end of a corridor then to floor 3 and right left and stuff like this in 1-person wide corridors and stairs. There I was scared of a fire because there was definitely a feeling of no way to escape fast.
Really? I spent a few nights in a capsule hotel in Shinjuku (Tokyo) and the experience was great. It was cheap, quiet, clean, and had everything I wanted (quiet private space, free coffee and "onsen" bath. Would definitely recommend.
Never stayed in one, but "quiet and clean" are pretty much de facto standard for any Japanese accommodation.
I always enjoyed their hotels, and my preference was usually some of the cheaper ones, like the Prince hotels. I have stayed in ones like the fancy Dai Ichi ones, but I felt that they weren't really all that much better than the cheaper ones (but I'm sure there's even fancier ones, for folks that can afford).
My biggest worry about these types of places would be safety. A fire could be horrifying.