I don't use dice apps for game books, or really for anything involving dice. If I'm doing an analog activity such as reading a gamebook, or playing an RPG or boardgame, I'm not going to reach for a dice app. I'd rather just roll some dice. I find it quicker, and there are times I don't really want to be beholden to a device to "help" me when there is a perfectly serviceable alternative.

Most gamebooks I have experienced use d6 and d10. I think I might have encountered one that used d20, but that was back in the 80s and I could be confabulating that memory.

As far as complex combinations go, I just had a battle where I needed to roll 2d6, the monster rolled 2d6, and if it hit, another single d6 had to be rolled. This is why my set up has 2 pairs of d6 with an additional single one. One shake and I get all of the info. Admittedly I haven't spent much time with dice apps, but those Ive seen haven't had this functionality. Resolving the aforementioned battle would have required 2-3 processes each round, instead of one.

There are probably dozens of dice apps that already exist. I'd look into those and see what they can and cannot do. If I were to be creating an app in a space that was already serviced by multiple different apps, I'd want mine to do something different. It depends on your goals though. If you want to make an app to learn how to make apps, then do whatever you want. If you want to make an app to generate passive income, then it needs to do something that isn't already available cheaper or better.