"subtle, hard to debug bugs" with START_JUCE_APPLICATION_WITH_CUSTOM_DELEGATE

I’m evaluating JUCE for an upcoming project with a non-trivial iOS UI, and it looks like the juce_gui Components won’t meet the needs of the project. It looks like I’ll need to create a custom app delegate for the iOS app. Looking at the source code. I saw this warning:

You can instruct JUCE to use a custom iOS app delegate class instead of JUCE’s default app delegate. For JUCE to work you must pass all messages to JUCE’s internal app delegate. Below is an example of minimal forwarding custom delegate. Note that you are at your own risk if you decide to use your own delegate and subtle, hard to debug bugs may occur.

Does anyone have any more details about what would cause these “subtle, hard to debug bugs”, and how to avoid them?