Three words to describe spey fishing the Niagara
Tough, tough, tough.
I've used a spey rod there, on and off, for over five years and it doesn't get easier. The Artpark drift has precious little backcast room and you can't wade out to get clearance. I worked it with a 15'6" 11 wt. and managed decent casts but didn't contact any fish. The depth of the water and the conflicting currents really confound spey casting and the steep shore screws up an overhead cast unless your head is very short.
The Whirlpool is more forgiving on the Canadian side and I've caught fish there on spey rods (the ultimate test). I'd overhead or underhand cast using short heads (don't try long line methods). Indie fishing with a spey rod also works quite well if that floats your boat.