Not Compatible With Popular Phones
Unfortunately, the Pip Boy Edition won't be compatible with every single smartphone on the market so you'll want to be sure yours will work if you're planning to use a phone inside the Pip Boy.
The developer says that because they "wanted to stay faithful to the dimensions of the in-game model, any smartphones larger than the models listed will not fit inside the wearable device." The Pip Boy companion application will still be available for these phones.
What this means is that popular phones like the iPhone 6 Plus, Galaxy Note 4 and more will not work with the Pip Boy. That's a big blow but it's one that you're going to have to live with because Bethesda probably won't make a larger model.