You have a RIGHT to be frustrated: the company set an expectation by heavily advertising an availability date that they wound up unable to meet. Supply issues are not an at-that-moment thing. Messages were going out from Angela Ahrendts close to the preorder date indicating to stores that there should be expectations of supply shortages.
The reality is if the executive in charge of their retail operations knows that far ahead of time, there's time to alter the corporate messaging to hopefully mitigate prior expectations from prior messaging. They didn't engage that, they're now being reactive, and they're dealing with frustrated customers who had the not-unreasonable/irrational expectation that an availability date of 4/24 meant it would be available on 4/24. It isn't a far leap from that, based on past experience, to assume that you might have at least 24 hours of reliable ordering before stock is sold out.
Apple either radically underestimated demand (which they quite simply don't get a pass on with their otherwise massive experience with demand estimates and industry-leading ability to ramp up production quickly), or they just plain dropped the ball on this. It's being made worse with the inconsistency with which orders are now being delivered (in terms of the "place in line" for recipients).
I recommend filling out the feedback form (for which you won't receive a response), as well as directly emailing Tim Cook (for which you MIGHT receive a response from someone on his response team) and expressing your frustration. Apple set an expectation of availability that they were not only able to actually meet, but failed to meet in spectacular and large-scale fashion.
There's a difference between being patient, and just giving a company a pass. One is a mark of maturity, the other is a mark of being a doormat.