Well, the rain really was insane while we there. At one point my umbrella got so wet that it just started raining through it!
Good and bad. It did certainly keep the crowds down I think, which shortened the lines as long as you were willing to stay through the rain. The problem is that after a few days of rain you run out of things to do. There are only so many times you can re-ride the same indoor rides. And with the rain it's much harder to really enjoy the theming - especially in areas like the HP sections. And good lord, the crowds in the stores of the HP sections was insane. The stores seemed extremely small to begin with, add in everyone in dripping wet ponchos or carrying around umbrellas trying to wait out the rain and the stores were impassable. We are HP fanatics and were quite disappointed that we didn't get to spend time looking at all the fun details. That's not Universal's fault of course (well other than the tiny stores) mostly a result of the non stop rain.
The funny part is how many of the 4D rides splash water on you as part of their affects. I imagine that would feel good usually, but it was pretty funny when you were already wet and cold. And since it is normally 90 out they have the AC cranked in the indoor queues/rides, so because we were wet and it was only 75 out, we froze during the waits. If you go during a huge rainy period I strongly recommend bringing a thin hoodie/jacket that you can take on and off as needed.
Also - most of the water rides were shut down when it was raining. Often there is thunder/lightning so they get shut down too, so the theory of "you're already wet just go for it" didn't work.
The Universal app did seem to do a very good job of staying accurate with what rides were down or not, and they did seem to try get the rides back up and running very quickly during the few lulls in the rain. This is where you wanna think about the poncho vs umbrella deal. We preferred umbrellas, but then when you could get on a ride after it reopened, all the seats were really wet (also bus seats were always wet) so your butt got wet if you didn't have a poncho.
I live in FL and this rain was really much worse then the average FL daily thunderstorm rain, but if you're there during a period like this I would recommend extra shoes as well. My kids packed extra socks, but they didn't think about a change of shoes. There are quite a few areas that don't drain well at all, and so you walk through significant puddles that can't be avoided. Your shoes/socks will get drenched.
It was a huge help to stay on site so that we could head back to the hotels and dry off and change before heading back out again.
Overall still a lot of fun, but 3 straight days of rain really had a significant impact on our touring of the parks. Again, though this was really not the typical Fl daily afternoon rain, so it was a pretty unusual thing.