Gah, that 'unbalances the crank' myth still persists?! Look at it this way, one one side of the crank all you have is a flywheel to resist the crank's temper tantrums. Lets pretend it's a new IRK setup that just needs the woodruff key and nothing else to work, the ultimate minimalist setup. On the other side you may or may not have a sling, etc.
"Oh Noes!" goes the crowd, "That's unbalanced, the sling is MUCH heavier and will cause havoc if you try to ride it like that. Now be sensible and put a proper flywheel on this instant, and a sweater because your mom feels a chill."
That's where most people stop looking at the equation. But what else is hooked to that overburdened side of the crank? Why, it's a primary gear, clutch, transmission, and ultimately the rear wheel! You're not going to fit a flywheel big enough to match the forces actually moving Lardo the Overweight Pitbiker puts on the transmission side as he has delusions of grandeur brapping around the track. It's an impossible goal to try and balance the forces out, you'd need a giant variable sized flywheel that would absolutely destroy the readability of the machine.
As to an IRK vs an ORK, how extreme of an effect do you want? With a lighter flywheel the motor spins up and down much quicker. You loose torque on bottom but gain HP on top due to less energy being wasted spinning up mass that is no longer there. Because of the reduced flywheel effect the bike will be easier to stall, and easier to bump start at the same time.
Crank whip is reduced with the ligher weight hanging off the end as well. I have a nasty habit of snapping cranks pavement racing so I run an IRK with no sling on the other side as a means of prolonging engine life.
An ORK offers similar benefits over a normal flywheel, just not as pronounced.
I have heard of more complaints with the chinese IRKs crapping out than the Daytona and Lifan ORKs, my Kitaco IRK has never failed me.
