You can learn something new every day on Lawnsite. Having been in the industry for 23 years and being on the board of the hydroseeding association and trading all the industry scuttlebutt with lots of people in the industry I thought I new most every hydroseeder manufacturer in the world but until you posted I had not heard of Henderson.
I did check out their website and took a look at their units. There are a lot of things I am not fond of in their design. The leg tanks they use would be one of the big concerns. I would never recommend a leg tank for a hydro seeding unit. The other concerns are less serious. They have one agitation jet going in the top. I can't see what they do with it inside but assuming they just have it pointed one way or the other inside the tank once the tank gets partially empty it's not going to mix. My other concerns are more minor.
There are other companies making units in those sizes that have been doing it for ages and are not all that different in price. The appear to be new. I think they have a few kinks to work out.