Based on previous things I've mentioned in my posts. The Volcano slides are the biggest risk of not opening with the rest of park (water coaster included). There's still heavy structural construction to complete, decorating, landscaping, and testing. And if that's the case, then you can bet the action river won't open either. You can totally tell they're behind where they should be by this point, and that certain pieces of the puzzle are holding up other contractors from coming in and finishing the job.
You think three weeks is a lot of time, but we're only twenty days out. Remember, this isn't a 3-acre attraction, it's an entire park, with a lot of things missing at this point. I'd even question a singular attraction's readiness if it looked like this 20 days out, and Volcano Bay is 10 times the size. I suspect they'll go full tilt starting this weekend and have hundreds of workers all over this thing up and until the very last minute.