The lack of consistency is due to each product releasing seemingly far apart from each design. But when I say lack of consistency, I don't mean in totality, I mean simply in virtue of looking at Eastern products that produce constantly new versions, so they have the luxury of thinking about each part of a stack.
I also don't think the power amp is mean't to be stacked with anything due to the heat a power amp produces. As far as the DAC and the HP4A, It's somewhat close, but I see your point (things like feat size and height really have no valid excuse in my book). Since this is a New York outfit (these devices, if finished with Matrix Audio levels of enclosure design, would be in the 5-figure range given the performance) you should expect some corner cutting in terms of the enclosure design and cohesion.
But I think Benchmark isn't concerned with this. They want to offer a device with Benchmark levels of performance, and no corner cutting on things like safety and functionality. I don't think they really care all the much about a product looking like it hails from an Italian design studio or something like that. If they did, they would make updates more often (the ABH2 is I think now over half a decade old). Side note, pretty interesting to see no one is even close on the SINAD charts in terms of power amps, even after all this time. There's a clear noticeable bump over ever other power amp measured. Why the industry is so nonchalant about Benchmark reigning like a King is beyond me (then again with the amount of shovelware that ends up doing fine on the market, it's not really much of a surprise).
EDIT: Forgot to include image of what I mean't when I mentioned Matrix Audio. Check out the milling work done on their Sabre DAC. This would be at an insane cost if you were going to get them CNC'd here in New York.