- Why 80mm wheels?

After testing different prototypes with different size wheels, I found small wheels have tendency to catch and do not slide as smoothly as bigger wheels.  The placement of the wheels out front serves many purposes.  It allows for any size wheels, big or small, and as much clearance from the ground as needed. Wheels placed in front of the foot pad instead of under mitigate nose dive better. I find that when I'm out riding and look down at the wheels, I get a distinct sense of the safety they provide, and I do think these are the most appropriate wheels.

-Why not bumper wheels?
The all plastic suitcase type wheels offer very limited help in nose dive mitigation.  Surface has to be as smooth as glass.  Check out the second video below of Slider testing on rough surface. You may wonder why they don't use bigger wheels.  If you look at your bumper and try to imagine the biggest size wheels you could squeeze in there, it would be no more than 37mm on the XR and no more than 33mm on the Pint.  There's simply not much room to work with.  So they use the excuse that they look 'discreet'.  Truth is, the wheels are just too tiny.  If you opt for front safety wheels, might as well get the most effective and legit device.  

- Is Onewheel with front wheels considered a 3 wheeler?
Yes, when you nose dive. At other times, it's still a Onewheel. :)

- Is slider something like "training wheels"?
The claim is foolish. Training wheels help kids learn to ride bicycles, and others to learn to ride electric unicycles. Slider does none of that. It is there when you need it. It improves your chances of nose dive recovery and if necessary will give you more time to run out a nose dive.
By the way, one of the popular features of Slider is the convenience of the front grab handle that is included with the kit.  In the end, the chances of a Onewheel nose dive are not insignificant, and from my years of experience, I know that Slider is well worth serious consideration.

- What about the front clearance with Slider and do the wheels interfere at all?

Slider wheels are mounted out front and that gives Slider a distinct advantage. It's even possible to use 100mm wheels and have as little as 1/2" angle reduction. That's less reduction than any bumper wheels in the market.
However, upon a nose-dive, it's the steepness of the angle that throws the rider as well as the momentum. After testing different angles, a slight reduction in angle along with good size wheels offer the best chance for nose dive recovery. There's plenty of clearance for normal riding, carving and going uphill.

- How much does Slider weigh?
While other products are either plastic or coming out of a 3D printer, each Slider is custom built from steel, wire brushed, primed and painted then mounted on and fitted to ensure alignment and ease of installation.  Arrives complete, no assembly needed.  Weighs about 1.5 lbs.  Installs in 5 minutes. Since most boards get a rear foot pad upgrade which is about the same weight as Slider, board balances fine.  Many boards now are equipped with an upgraded battery which weighs more than the stock battery and Slider will counter balance that as well so it doesn't feel tail heavy.  The newest Pint X has double the size of the original Pint and Slider will counter balance that perfectly.

- Is Slider effective off-road?
Slider is meant for pavement although you're probably not better off on stock bumper off-road. Truth is, no front safety wheels device is 'effective' off-road due to too much drag.  Personally, I keep it on at all times.  However, it takes few minutes to remove for situations where it may not be needed.

Below is an example of a nose dive slide-out.

Check out multiple videos with Slider on my YT channel.