Hello and welcome to the board! Your post in the Dry Fly thread nails some advantages for Tenkara over a fly rod & reel in small Tenkara-friendly streams where “Tenkara” was created and flourished that I’ve experienced and demonstrated quite a few times fishing alongside a buddy who fishes a western rod but has yet to become enlightened…
When I began fishing with a Tenkara rod I wanted to expand the use of a fixed line rod for big rivers, and saltwater.
Honryu is a fixed line discipline that I’m not very experienced with but would like to be successful at. I know there are some rods specifically designed to reduce the problems I’ve had with the application of a fixed line rod to “Big water, big fish”.
Off the top of my head disadvantages I’ve found with fixed line rods for BIG water include:
- Even with my 5.4 meter Keiryu rod and a 5.4 meter line my casting distance is limited to around 8.2 meters to maintain the “Tenkara Triangle” rod-line-fly geometry. On some days under specific conditions it works. But often for BIG water in high flows that limits wading, and especially in saltwater, 8.2 meters does not reach out to where the fish are holding.
- Increasing the line length with a level line makes it harder to cast, especially in windy conditions that always seem to be present in the saltwater environment.
Using a longer furled or PVC line can make casting easier but I believe…
- Carrying more line in the air on the back cast increases getting snagged up and tangles.
- Control of drift and manipulation are not as precise, and strike detection and hookset will not be as fast or effective when the geometry of the of the rod-line-fly “Tenkara Triangle” changes.
- Landing a fish is much harder handlining in the extra line length.
- The Rotational Moment (tip heaviness) and Moment of Inertia (MOI) of a robust, longer rod is going to increase making casting more tiring as the day goes on.
At some point in BIG water, the ability to shoot compact heads longer distances to reach fish and the ability to reel up line to land a fish overcomes the reduced efficiency and effectiveness (fun?) I have with a fixed line rod. Each of us must individually determine where that point is. I’ve not given up yet but it’s something to be aware of. 
I do find the ability of flexible fixed line rods to protect lighter tippets with hard fighting wild fish to about 18" is astonishing.