I agree that it’s game and situation dependent. Flailing on a game where an upper flipper is important in multiball is a bad idea. But there are some games where I say flail away.
For example on LotR, in TTT multiball if you’re getting double or triple jackpot 4 and 5s, you’re going to advance through a lot faster than cradling up and hitting single jackpots. Although once you’re down to 2 balls it’s probably a good idea to trap, but that’s a multiball that kind of encourages controlled flailing. On the other hand if you’re flailing through fellowship you’re gonna have a bad time.
Similarly on spiderman, unless you need a specific villain or are trying to bring in another multiball you can do decently hitting green goblin and sandman with the flail. Again when I get down to 2 I try to trap up and shoot venom shots or lock targets or whatever. But if you try to flail through battle royale it isn’t going to last very long.
That being said, if I had the accuracy, recovery, and separation skills of a pro I would probably trap up every time without question. I’m going to bet on those skills all day if I have them. For average players like myself I think flailing is ok sometimes, but you definitely need to think about what you’re trying to accomplish in the multiball.