I don’t know what answer you’re looking for besides the one you’ve been getting. It's a rehearsal, which is a completely valid reason. If there are any issues that go beyond this, Universal has made efforts to address it by offering something in return. Anything else is simply online noise.
Any sane person who expected this to be the #1 park in the world from the get-go was setting expectations too high. IOA and USF had issues. DisneySea and Disneyland had issues. The parks will always grow and evolve beyond the opening day stuff, which will be just blips in their long history.
The rain issue is valid, and there's only one way to fix it, and that won't be done by May 22.