Hi Charly,
Since the beginning of summer, I have a Think Nitro. Same boat as the P&H Octane, surfski version of the Valkyrie.
I bought it for exactly the same conditions you mentioned: rough water, shallower rivers, .. basically anywhere I wouldn't want to take my Nelo550.
The one I bought was an unsold surplus that required some work on rudder, ...since it had not been put together by the dealer yet.Long story short: it is really half-way between sea kayak and surfski in terms of performance.
Whereas I can maintain a cruising speed of about 8.5 km/h in my sea kayak and 10.5 km/h in the Nelo over an hour, I hit about 9.5 in the Think Nitro. The stability is very similar to the Epic V7 I used extensively a few years back.
I find it quite nimble and well-mannered in waves, but I have an understern rudder, whereas the Valkyrie comes with an overstern. One thing is that the rudder lines show quite a bit of flexibility. The line are the Nelo rudder lines and very stable, so the flexibility must be coming from the hull bending or something with the internal guides. I doesn't bother me much, but YMMV.
Being PE, it is obviously quite heavy, but that also makes it a perfect loaner boat for friends and family.
Since the Valkyrie shares the same hull, I would expect it to perform similarly w.r.t. speed and stability. The storage space of the Valkyrie, from what I saw at the dealer, is very similar to a more traditional sea kayak, but you are likely not going to fill it up to the brim with camping gear when going for a playful paddle in rough water...
If you want to see the Valkyrie in action, I would recommend the video's of NorthSeaKayak on youtube. Dimitri is a P&H team paddler and is using a Valkyrie since 2018 or so... He also wrote a review of it on P&H's blog, which I think is well-written.
So, all in all, I wouldn't call it a slug, especially in view of storage space and ample stability, but it is not a race boat either. Horses for courses.
Best regards,
Tom