When some of the slides have a capacity of under 200 per hour (ironically, the biggest most iconic ones besides the hydromagnetic one- which is excluded from express anyway), then it's very hard to allow paid line jumping. If they allowed passholders to line jump as well, it would just exacerbate the problem. I still think this will be the biggest unforeseen challenge that Universal will face with the "no queueing" aspect when it's combined with the "paid line jumping" stuff. Imagine this scenario:
Slide allows say 1 rider every 20 seconds (as the big iconic one in front will likely do). Universal will probably have 10 minute windows which means about 30 riders can go during that 10 minute slot. If all 30 show up at the same time, and an additional 30 people show up with Express, then there are now 60 people in line. Since it takes 10 minutes to get 30 people through, the person at the end of that process, despite being promised "no queueing" is in fact going to have to queue for 20 minutes. It's just unavoidable. At a water park where capacities are usually in the low hundreds, I think it's going to be extremely challenging to schedule enough people to keep the slide full, yet not ever having a queue especially with express.
Maybe they'll do away with time slots and just "call you" when it's time to slide with no real definitive timeframe during booking. Even then, they'll need to call 30 - 50 people at a time and hope they all arrive in a staggered order. Then they'll also have to hope 50 express people don't walk up before the timed entry people arrive.