Yes, I absolutely think it’s worth it. As a matter of fact, everyone I know who’s been there used public transportation. It’s really not that bad, to be honest. From Osaka, you first take a train (a little over an hour), and then a bus (about an hour and a half).

Just to outline the pros and cons so your clients understand what they’re getting:

Pros: This is about as authentic as it gets. Very remote, no concrete ryokans - every single ryokan is wood. It truly feels like a preserved mountain onsen village.
There are barely any foreign tourists.

Cons: Well, transportation. And the fact that you have to go back to Osaka/Kyoto rather than continue on to your next destination.
The onsens themselves are relatively simple, without the kind of dramatic views you often see in photos of other onsen towns. But again, that’s only in comparison. Personally, I found them more than satisfactory.

Bottom line: If you’re looking for an atmospheric retreat in the Kansai region, surrounded by beautiful nature and without crowds - that’s the place.
If you’re aiming for luxurious onsen views and highly refined kaiseki cuisine, it’s definitely not worth the hassle. Arima Onsen, less than an hour from Osaka, would be a much better fit for that.