I can't imagine a person who just spent $12,000 on a 600mm lens having anything good to say about a new 600mm lens that costs $1,200. You can be sure that each lens has advantages and disadvantages. If somebody would give me one lens or the other I would choose and suffer with the disadvantages of the $12,000 lens.
It has nothing to do with the fact that I bought a 600mm f/4 lens. It's a logical deduction. If you are going to buy a 600mm f/5.6 prime, the general expectation is that it fits the general description of a prime. You don't expect any prime lens to offer lower quality than a similar focal length from any zoom lens. Most people would natrually compare it to the best of the relevant zoom range, in which case the Canon 100-400 L, which has very little CA at 400mm, is the likely target. Any 600mm f/5.6 prime would have to be better than the 100-400 L @ 400mm.
And, as a matter of fact, I do have good things to say about Tamron's lens. For it's price point, it's a game changer. It offers excellent features and great quality relative to it's competition for an eminently affordable price. It's because
I have good things to say about Tamron's lens that I do not believe a 600mm f/5.6 prime lens is viable.
There really isn't a middle ground option to be had. Building a good quality 600mm f/5.6 lens, especially if you cut some corners on weight savings, optical quality, etc. is too expensive, it would be around $4500 at the cheapest, and $7000 at the most expensive. Simple fact of the matter is, you can pick up a used Canon EF 300mm f/2.8 and a 2x teleconverter for LESS than $4500, and you have yourself a 600mm f/5.6 lens that offers superb optical quality (the old EF 300mm f/2.8 was once called the sharpest lens in the world). If you want even better quality than that, you can pick up the new Canon EF 300mm f/2.8 L II for $6800, and have the best 600mm f/5.6 IQ your going to get from a lens at any price under $12000.
There simply isn't a place for a 600mm f/5.6 prime, not one that lives up to the expectations customers have for "prime" lenses. The option, with teleconverters, already exists, and even used, it tends to be well above the price point most consumers who are looking for a "cheap" 600mm lens option are willing to buy at.