I'm personally more skeptical of the Zettelkasten/Roam/PKB craze as being effective, but that's only because I feel like I've had yet to find a problem where it would have been the solution. I'm constantly left feeling like it may be the solution to many problems I have related to memory and remembering thoughts, but without personal experience in it, I can't say what it's worth.
I don't think it's even possible to evaluate a productivity system without using it for a long amount of time and conducting periodic reviews on it regularly to see if it's helping with anything. I don't know what the metrics to measure are, and I don't know what a review like this would look like.
On the paper journaling thing, though, I think the answer is much more clear cut: it's more about the artistic nature of it than anything else. A paper journal is almost always impossible to search, easily damaged, easily lost, and easily customized with stickers, different pens and pencils, and other decor. It's easy to find a notebook you like with a paper you like and then justify writing in it. However, I don't think there are many practical benefits that can't be applied to an electronic system that's functionally similar, save for the effect of slowing down entry so you remember it more.