As an end-of-the-day scenario, it's not bad.
As a "Ha! Ha! We punked you!" scenario earlier in the day, it's terrible. Family thinks they're going to ride Space Mountain, but then can't. Being given another FP (that they've already learned they cannot count on) will not be a huge bonus.
If the ride breaks down, I think people should be able to ride it when it comes back up.
I really think that if these FPs can't be relied upon, it will totally backfire for Disney.
And if I were planning and knew that they couldn't be counted on, I'd plan like they didn't exist, with Space Mountain first, Splash second, etc. no way I'd take a chance on missing Space or having to wait 80 minutes for it.
Also, knowing that they can't be relied on, it would never be seen as a perk to staying in a wdw hotel. For me, anyway.