There's always such issues with water rides, too. You just need to look at other water rides in history to understand that they're highly likely going to experience some form of technical issue - especially something of this nature.
The problem is that water is chaotic and, crucially, rather heavy.
It's predictable in the sense that it will flow downhill, but quite how it does that and how fast it goes and how turbulent it is is not particularly easy to work out (the equations that tell us this stuff are famously unsolved - we can just do half-decent approximations -
https://en.wikipedia.org/wiki/Navier–Stokes_equations).
The problem is that it carries a lot of momentum, and so makes it difficult to work with and it'll bite if it goes wrong.
Throw some large rafts and great big 'elements' in to the mix and you've got an awful lot of uncertainty in your design, which means a lot of testing, a lot of tweaking, and a lot of faff. Especially if that tweaking requires the water to be drained down each time.
Lot of sympathy, and respect, for the engineers trying to pull this off.