My main issue with the Rivals challenge this time is it's the first time that I can remember an event/challenge/whatever that requires you to use rushes to even get halfway through, let alone complete it. Unless, of course, you have at least 50 production buildings in your city*. That's a marked departure from everything else we've seen. Sure, you could use rushes in other events/challenges to get through them faster, but never just to be able to finish them. My favorite word for most of Inno's changes additions to the game lately: RIDICULOUS.
*If you had 50 production buildings and already had them ready to collect from a 24hr production, you could collect from as many as you need for the first task that requires them, then reset them. Do this each time and you could do it without rushes. The only way you could do it without rushes. RIDICULOUS.