American side is flatter than the Canadian side overall imho.
The American side isn't just flat, it's all on the upper side. You can't even see Horseshoe Falls from the US, just the crappy little American Falls. The US side is a sheer cliff the whole way. The Canadian side ranges from lower lake, to upper lake (top of the falls), to the hill that rises away from the water, up, and above the falls.
The downhills he's talking about are mild and long, it's not braking on the edge of a ravine. Old ladies and their walkers go up and down them around the casinos:
I find it hilarious that people with DUI's and such, or SouthAm/Euro/Asian tourists who only got a US visa, and can't get into Canada, thinking "well we'll just look at the falls from the US side", and they go out on that expensive observation platform that reaches right to the edge of the border, looking back at the crappy American falls/Bridal Veil falls (which aren't even 30% waterfall, they're like 45 degrees down to the lower lake most of the way), and are like "Oh wow! It's so beautiful, it's so amazing. Niagara Falls is a wonderful place!", and have literally no idea what's just a thousand feet farther down the cliff. The geography is such that you just barely, just can't quite see anything at all of Horseshoe Falls.
Anyway, I digress.
They're not rappelling over the edge on the US side. Braking is a non-issue with ordinary OEM brakes.
The only reason there's any braking concern is because the "brakes" on a Model T are a greased up cotton rag pressed against the transmission, and they're basically a suggestion. See SuperFastMatt's breakdown of the Model Y vs. Model T:
(Relevant section is at 10m30s)
You can do resistive braking instead of plug braking on the DC motor. Plug braking is basically putting the car in reverse and feathering the throttle. You're actually spending energy, to lose energy, so it dumps twice the heat into the motor than you actually remove from your momentum. Resistive braking is simple, but less powerful. Just a big dummy dump load. oven or stove or dryer element, not sure what the right ballpark would be. You'd want a heat shield separating it from the wooden car body though. It's only 10x as much heat to dissipate as an adult on a bicycle. It's really just not a lot of energy.