If you don't like the Soiree, then there is no need to buy it. It's a free market. As a practical matter, you pick the Toto that looks nicest to you, in the right price range for you, and it will work fine. Don't worry about the engineering.
I can see why you think what you think, but I think the only point that's sort of valid from an engineering perspective is that you have to spend an extra 10 minutes to install two extra screws.
I'll give you just an outline of my thoughts; I am sure others will fill in in more detail:
(1) No flush goes "straight down". All toilets have a trapway, because there's no trap in the waste pipe.
(2) Modern trapways for Toto low-flow toilets are designed with curves specifically to prevent clogs.
(3) The Unifit simply replaces a portion of the china trapway in the toilet with a floor-mounted portion that can come in three lengths.
(4) Far from being prone to "potential clogs" in comparison to something that goes "straight down", Toto's trapway design is considered the best in the business and they certainly put more resources into trapway design and redesign than anyone else. It's the design of the siphon and trapway that enables them to to do a 1.0gpf toilet coming on the market soon.
(5) The weight of the toilet is indeed straight down. Nothing is cantilevered. I think I see what you're trying to say, but I don't think the engineering works the way you are thinking it does. The floor bears the weight of the toilet evenly along its entire length; the weight is not concentrated on one point.
(6) You seal up the connection to the waste pipe nice and tight, so no leaks will happen there, and the male-female connection in the rear is nice and tight as well. I have no concerns about my Carlyle II leaking. None.
If you want the Soiree but are concerned about the engineering of the trapway, don't be. If you don't much like the Soiree, pick another Toto. In either event, enjoy the flush!