Reason for the high ratings is as follows.
Graphics: Best Live2D you can get in any mobage. Beats out even games like Azur Lane or Shiny Colors in this aspect. The card art is good too. Uses chibi models during actual gameplay, but it works well enough.
Sound: The sound is good. The music overall is pleasant, the VAing pretty nice, and overall is just good.
Gameplay: It's fun as a shooter semi schmup. The game can actually be pretty challenging depending on the boss and how much power level you have under the boss. Bosses are varied enough, from giant mechanical spiders, fox girl, katana lady, and ship. Course the usual loop outside of your challenges is grind as always in mobage, but it has an amazing QoL function that lets you repeat a map a number of time or until you are out of sanity. Would prefer skip tickets, but you can't have everything. If there's a mode that I want to come back, it was the battle royale pvp mode.
Storyline: It's a good waifu game. The system reminded me of BGHS with the character stories, though they seem more intent on adding onto this. The additional text messages help to drive in the personality of the girls, an effective interaction mechanic with less cost. The story can be a bit weird at times, especially considering the MC, but he's likable enough.
Value: This is the most important part. As a player who has been playing since the start, the 50 roll pity, 100 roll pickup pity, and 300 spark helps you not get screwed over in the gacha. The game throws many tickets for gacha. Currently 100% collection on limiteds. There are just a ton of sources for rolls. There is also a weapon gacha. This is normally a negative, but they give you rolls for it constantly, including the daily so I am willing to turn a blind eye. In addition, weapon spark is only a hundred rolls, which is really easy to save for.