That's a pretty partisan view.
Here's a non-partisan one from someone who isn't in the business:
1. Video cameras are not "worked" like a still camera. Specifically, some controls - most particularly zoom - are often required to be used while the shot is being taken. Yes, its not good to zoom around willy-nilly in video, but at times its important to be able to use it during a shot. A still camera has exactly one button to press at the moment of truth - the shutter. This is not true for video.
2. Viewfinder access is a BIG DEAL. Dive with a visible screen and you will understand. This is easy with a still camera or even a still camera in "movie mode", as the screen is on the back. This is NOT simple with a video camera. A housing that is big enough to have the screen open facing backward has two problems - volume (meaning positive buoyancy), and the screen is way forward in the housing. A mirror arrangement can partially solve the problem but not completely do so - especially since Sony started turning off the "reverse" when you turn around and then close the screen! An inverted image is a PAIN IN THE BUTT to use on the viewfinder. "Big eye" viewfinders are a possible solution, but require you to keep the camera up high to be able to line up the viewfinder with your eye, as does, to a large degree, mirror arrangements. I like to swim with it BELOW my body so that I am safely away from the reef/wreck/whatever - this means I need a MONITOR so I can see what I'm shooting.
3. I want my "shooting controls" where I can get them without having to take my hand off the grip. These are (1) REC/STBY and (2) Zoom. Ideally I also want manual focus on the grip. The rest can be somewhere else, but those three reallly need to be where I can get to them without having to fiddle-fart around. Mechanical housings have problems with this since the buttons end up wherever they end up - the housing maker has little control over it due to the nature of the beast. It becomes a REAL problem for a video camera as the "shoot" button is USUALLY on the back, and the grip is near the front! How do you put that where I can get it without taking my hand off the grip? Its easy with a still camera, since the shutter is on the front right, where your hand is with a housing anyway - so the lever arrangement works really well. It doesn't nearly as well with a videocam.
How many digital camera shooters underwater use the monitor as their primary (or even only!) viewfinder? MOST IF NOT ALL! Yet this is IGNORED for most mechanical housing folks when it comes to video. Why? Cost. The monitor back is expensive and mirrors don't work well since the monitor isn't up against the back of the camera. Ok, so do you want a working solution or half a solution?
Both mechanical and digital controls can fail. Designs vary. If mechanical controls fail, your housing floods and your camera is destroyed. If electronic controls fail, provided that the grip part fails you still have a working system - turn it on, put it in record mode, and go dive. The better electronic housings isolate the electronics from the grip with a wetplug or use magnetic switches where there is no penetration in the housing at all - there is no way for a grip failure to screw the housing as there is no path for water to get inside the housing through the wiring.
Now you can solve most of the mechanical "issues" with a monitor back. But now you're up near or even past the cost of an electronic housing.
Finally, however, you have one serious problem that no mechanical housing can solve - interchangeability. With still cameras we are stuck with this because there is no common command protocol for electronic controls. This is NOT true for video cameras - at least not for Sony cameras.
Obsolescence protection is a BIG DEAL, and its one that VERY FEW housing manufacturers pay ANY attention to. You SHOULD, because with the pace of electronics today, the camera I buy tomorrow won't be available - at any price - in a couple of years! Not only could I damage the camera on land (or simply wear it out or have it break out of warranty, and have it be uneconomic to repair) but I could flood underwater, and even with insurance the camera may not be available - but since the housing was not destroyed I will be stuck with a check for the camera and an unusable housing!
Ike, this is where you lost when I bought my video camera housing. You didn't lose on the mechanical argument - you lost on the upgrade/replacement path.
In short, investment protection.
USVH will swap the physical plastic (the actual "housing" portion) for a VERY reasonable cost if I upgrade cameras in the future and the new one doesn't fit. SOME of the cameras I might upgrade to WILL fit though, so I might not even need to spend ANYTHING with them. But even if I do, its reasonable - I was told $300 or so, plus perhaps (depending on the particulars) another $100 worth of labor.
With the mechanical housings I throw away the entire investment, and I still need to buy the monitor back - at least I do, for the way I like to shoot.
I was FORCED into this paradigm with my still camera, but I'm NOT forced into it with a video camera, and this is one game I won't play unless I have to. I don't have to with the video camera housing options out there today!