At MasjidNow, we have previously received requests to build custom apps for mosques. We've always refused the offer, however, because we don't believe that it is a good idea.

It will be expensive

Mobile app development is a fairly expensive proposition, as opposed to website development or purchasing a subscription to a service like MasjidNow's. A mobile app developer can charge in the hundreds or thousands of dollars to build an app, which doesn't include future updates, bug fixes, etc.

It will likely be unmaintained

As stated above, if you don't constantly pay for updates, then your custom app will not recieve any new features, bug fixes, etc. and will likely fall out of use. You are then stuck with an outdated app on the App Store that reflects poorly on your organization and is not useful to your community.

It will not be well-made

This is simply a result of any contracted software product being led by the inexperienced. Unless the mosque's webmaster or technical lead is also in the business of designing and leading the development of mobile apps, then the final product will be the result of an inexperienced person paying for the (expensive) work of someone else who is not interested in creating the best app experience.