I've wondered about this a lot too, and the answer I've come up with is that it'd be fairly variable depending on the person, as another anon pointed out: probably incredibly effective against certain people, much more effective than you (or they) would think against most people, and probably useless against a fair amount of people as well.
Angelina is a good example of an absurdly ticklish girl that could be pushed to a genuine breaking point if she was submitted to intense, real, tickle torture for long enough (vidrelated I believe should be sufficient evidence). Plenty of other models, mostly amateur ones for obvious reasons, have (what appear to be) genuine panicked reactions during even small sessions, likely owing to traumatic past experiences relating to it from their youth (which are more common than you would think and in some cases a genuine form of child abuse) that make them more susceptible, or just natural outrageous levels of sensitivity, both of which would probably make tickling just as effective if not more than other 'traditional' torture methods on them.
As for it being a lot more effective for most people than they'd think, I'd refer you to the Tickled documentary, specifically this interview with David Farrier, the creator, who I believe puts how a random, unprepared, averagely ticklish person who is not in on it at all would experience it best:
Here's him talking about the experience in a Q&A, he sort of extends it to his crew too, which gives us a larger sampler of normalfags being tickle tortured to consider:
I think the last part is what sells this point for me, the idea of 'you think you're not ticklish, but then you're tied down in a room with a person whose profession is to tickle, and it's really bad' (paraphrasing that last bit).
Lastly, while I believe every person is ticklish to some degree, some are really good at keeping it together and sort of blocking the sensation if they set their mind to it, which means tickle torture would be very ineffective against them. I should know, I'm one myself, and I've also encountered a few other people who can easily reduce it to something that barely makes them smile. Funnily enough, people like this can also let go of this self-control if they so desire, at which point the sensation overtakes the rational part usually in control and makes it very hard to pull themselves together and stop it from affecting them again while being tickled (this may or may not be the case for yours truly) so... There might be hope of breaking even for these supposedly not ticklish people if you can get them to drop their guard down.
There's also, you know... Tickle sluts, who you might be better off torturing by witholding their tickles. But then again, those are hardly the average right? With the exception of course, of our lovely board~