Movement is the concern from some I've had good feedback from. Studying and living with the joint closely, I'm pretty confident the cabin and deck stay together at the joint.

Here's a sketch of the mechanics. The deck has an upturned flange molded in that is screwed through from the inside the cabin(twice, there's a handrail also fastened through). When it was assembled, screws also went up the center of the cabin sides from below. There are also drifts as you would see in a wooden cabin, running from under the deck, through the cabin side, and also through the plywood top(all these fasteners were then glassed over, and the boat finished). It's never leaked water through this joint. Rather the problems were more the result of the trim piece covering the bedding, and water ingress between the trim and cabin.

My wooden boat adviser (at the boatyard) thinks this will work as it has on wooden boats they've done, some of which have gone more than 20 years without any work needed on the fillet. All those decks are glass on plywood but otherwise a similar situation.

I'm a little apprehensive(at first, some years ago, I didn't like this idea, then I watched it for years...) but I think this may be a better long term fix. I'll let you know.