Fallout Shelter is also an interesting case study in microtransactions. Josh Allen's criticism (from his DGYHU podcast) was that they flubbed the microtransaction model, or maybe shouldn't have had them as the game was mainly an ad for Fallout 3. (Caveat: if there have been any significant changes to the game since I played it, I probably missed them.)
In Fallout Shelter, your goal is to help settlers survive in the wasteland. To do that you have to manage your base and build it out. As you build out your base, threats increase and survival gets harder, but as you level and equip your settlers it gets easier to handle these threats. Once you have a lot of high-level geared up settlers, base management is easy and you might feel like you're done with the game because you aren't making decisions. An abusive business model would have been to make it very slow to level and gear settlers without microtransactions, so that the player feels like they are drowning unless they spend money. Microtransactions could also have been necessary to correct mistakes in your base build-out without starting over. In this model, the game might not have been very fun for players who didn't spend money, which might have made it a bad advertisement for Fallout 3.
So instead, the game flows pretty well if you don't spend any money. If you do spend money, you reach "the end" very quickly and you feel like the game is really short. I think this became even more the case when the game introduced robot units (I never saw these), as they take care of the easy management busy work for you, leaving you with a completely self-running base.