I install a lot of hallmark. And have done so with the walnut.
Great floor. From an installers pov. Its nice. Not too many issues with product.

As far as comparison between solid versus engineered? Both have advantages

Biggest drawback is yes walnut is softer but with solid it can be refinished many times (depending how qualified the person sanding is)

Engineered can usually be sanded 2 times (depending on the person sanding.

Thanks for your response! You are the first person I have found who has actually installed the Hallmark Walnut. Would you say the engineered is just as soft as the solid walnut? Or is it more durable due to the finish on the engineered product?

Walnut is Walnut. A Walnut tree could go one way and end up as solid flooring or go the other way and end up in an engineered product. The hardness and general wood performance would be the same.
Prefinished products generally have a very durable finish compared to site finished floors which would make them more durable.
Engineered products would generally be more stable than solid products. This factor is especially important when considering a wide plank.
I have a Hallmark display but haven't yet sold or installed any. It looks to be a nice quality product.

Hallmark is pretty decent. By no means the absolute best but its nice. Things i like is i usually dont install less than 400ft of it at a tome. Not as a rule that just seems to be the lowest amount ive installed. But honestly i had to only scrap two or three boards due to color issues or damaged grooves.

As far as stability goes. The engineered will flex less if that makes sense just because of the way the plank is assembled but the top 1/4in is solid walnut and that dents and scratches just like anything else walnut. The finish is strong and durable but not scratch proof.