You should write in your contracts that UI components will work according to a guideline for IOS that guideline should be Apples HCI document which is complete

You should write in your contracts that UI components will work according to a guideline for IOS that guideline should be Apples HCI document which is complete and their components conform to it. Anything beyond that is out of scope. This is why when I do UI work it is always hourly based, when it comes to the UI everyone is an expert because they have an opinion about what they can see. Some of them will even ignore blatant research and force you to conform the interface to their opinion. Taking UI work on anything other than hourly is a dangerous proposition because it is where an overwhelming majority of changes are made. The other area is adding completely new features, you have to isolate both of those from a fixed price contract. New features are pretty easy, if it is not in the spec it is a new feature. UI is more difficult it is subjective that is why having a third party guideline as the definitive agreement is an important arbitrator. If they fail to read it that is on them, further call out an hourly rate above the contract for customization and new feature work. Finally, if you have a client that does not understand that you deserve to make a living wage for your effort you need to walk away, there are many people that expect others to work for slave wages for some reason. Counseling them in fairness tends to be a moot point and burns up more of your time, as soon as you spot the tell tail indicators that you are working with someone that views others in this light, you have to walk away no matter how deep you are in, people such as that will see that they are getting more than their fair share and milk the situation.