To be sure your app will be validated by Apple you have some things to know (for Android’s Google Play there is a validation too now but, it’s more permissive):

• All your pages have to be filled out with content. Please be sure you have no empty pages. If you have a news feature for example, enter one news, and with a picture it would be even better.
• Content: no explicit contents in your apps! But it was obvious.
• Design: please choose cool colors with complementarity. If you don’t really know how to find colors with complementarity you can use http://paletton.com/. But no rainbow apps, please don’t.
• Features: Apple wants useful apps. So don’t create apps with only one or two features. And enter relevant content with multimedia (videos, images, etc). Siberian integrates many user-facing interactive features for your apps which help to build a useful app, such as the Commerce feature, or the Realtime Chat feature for example.
• Write a good description of your app. You have to explain what the users would be able to find in your application, and why they have to download it. (250 characters minimum).
• Never talk about other platforms in your app! Apple doesn’t want you to talk about Android or Google Play, or Samsung in apps that are on the Apple App Store. Even if there is a logo, or an image about an Apple competitor, your app will be rejected.
• If there is an account registration tab in your app, or part of your app that is blocked with the Padlock feature, be sure to provide test credentials to Apple in the “Sign-In Information” fields on itunesconnect.apple.com. To know how to create test account, take a look to the Padlock feature article.
• Never use test content in your app, like “Lorem Ipsum” content or texts with “test”, “demo”, etc. Apple refuses apps that are not finished, and/or apps for tests.
• Regarding the new Apple rules, we advise to customize your app as much as you can, so that the apps don’t “look and feel” the same. Another very important point is that you better publish the apps under the developer account of your clients and not under your developer account. That means your clients would have to take their own developer account, but it’s safer to proceed like that.