It can be amazingly tough to stop these dogs when they're in high gear, especially when game is involved. That becomes a high-blood-pressure moment when the target is a skunk or porcupine!!!
The little scooter's brakes left a lot to be desired, although they worked OK for 2 dogs if I also drug a boot. I think I actually melted the rims a bit on mine before I gave it away to a kid.
There's a ton of debate about traditional X back harnesses vs the new shorter harnesses, the latter of which are designed with skijoring or scootering in mind. They certainly seem to work for some people so my story is just one bit of info to consider....
I'd always run X backs but decided to try something new. I had a set of short harnesses custom made for Los Tres Horribles, got them all adjusted properly, then hooked them to the #130-140 lb cart headed uphill. This is a standard run close to the house we do frequently... I'll get off to run behind if I have to, but the dogs have to tow the cart regardless.
Within the first 100 feet it sounded like I had a team of asthmatics, and the power loss was quite noticable. I stopped the dogs on the hill, which meant they had to keep leaning into the harnesses to keep the cart from rolling back so I had a pretty clear picture of how the harnesses were working.... which was, in one word: DISASTER. Every one of those harnesses was pulling too hard from the attachment on the back, riding the collar section up, and digging into the throat of the dog!
I tried adjustments ad infenitum, I tried even sending the harnesses back and having them re-fitted and going through this it all over again. They never worked right on all 3 dogs no matter what we tried.
My analysis: In looking at the classic X-back design, you'll see that there are straps going uninterrupted from both the top of the collar back to the tug, and from the bottom of the collar through the chest plate and back to the tug. When an X back harness is properly fitted the pulling pressure comes in on the bottom AND the top of the collar, but this pressure is equalized, and it doesn't ride up or down on the dog. The problem I see with the "short harness" designs is that the pulling all comes into the top of the harness with nothing to hold it down from below, and, logically, will pull that collar section up.... with results like what I saw.
I always have at least 8' of line from me or the scooter to the first dog with a fairly heavy bungie section in between, so any 'lift' on the harness is very, very minimal (if it exists). I never have been convinced this is much of a problem to solve... I tried the short harness mostly because people were raving about performance improvements with them. So much for that idea.
I'm currently using a harness from Alpine Outfitters called the Odyessy... it's a little hard to get on and off but so far I think it fits the best of anything I've tried.
FWIW from the pictures I've seen, it looks like most of the Iditarod guys are back to using more X back type harnesses after a period of experimentation. An interesting point I've seen mentioned here and there is that the short harnesses actually encourage the dog to pull LESS, hence less soreness and the dog not tiring out as soon in a big team... if true, this is quite impractical for those of us with very small teams where all dogs need to be pulling their share most of the time. For an interesting take on this and Mitch Seavey's completely opposite approach- using a design with spreader bars in the harness and gangline that almost seems to hark back to the Gold Rush- see this article from Cabela's Iditarod coverage.