This A380 Evac test thing is a complete red herring. I have been looking at the 747-400 Upper Deck slide. It is a double slide securely attached to the door, and non detachable, with large inflatable side fairings near the top. The angle is fairly shallow. In the event a 747 tipped on its tail (in the event of loss of body gear), Doors 1 slides at the front will be unuseable, but the Upper Deck slide will still be perfectly useable. In general using this slide may be slightly riskier due to the danger from its increased length of twisting (in which case don't go down it!), but I would as happily go down that as a main door slide. Now, I don't know how much higher the A380 Upper Deck slide is than a 747-400 one, but I would guess it is not much more than a few feet. So, if 747 Upper Deck door slides have been happily used and they don't have a reputation, where is the problem? I think it is a case of 'put up or shut up'. The designers know what to design for- we should let time prove they got it right.
If the designers of that waterpark in Orlando got their 'Stuka' waterslide (a seemingly near verticle drop on a concrete gutter) right, then who are we to question?