What are the concerns, if any, in using a code word for a lengthy prayer, like a Mega Prayer, or even a group of prayers? Will that be just as effective?

Saying a prayer, or a group of many prayers, and assigning a code word for the entire grouping to which additional prayers can be added over time simply with a request, "to add this to my group given the code word X," will be just as powerful as the original launching of the prayers individually. The reason is quite simple. Acting on the code word, the divine will go back to the point in time when that prayer, or group of prayers, was launched and will relaunch them exactly as done originally with the same level of intention in all respects. Thus, an identical repetition is guaranteed. So the only liability is to say a prayer once and assign a code word, or a group of such prayers, and then only to use the code word, freezes in time the launching, with respect to the quality and level of intention behind the launch itself, based on who you were in the moment saying the prayers, your level of knowledge, insight, passion, and zeal. People can grow over time, become more enriched, develop deeper insight and concern about issues of long-standing, and thus a prayer launched sometime later might have a stronger intention than the first outing. This is why we recommend that prayers be repeated, even when given code words, at least at intervals. The repeats can also be added to the pool, and that will heighten the energies as well and will be a perfect way to further amplify the benefits overall.