In my time teaching in the Sim....from the early 76 Simulator at American Airlines Training....to the very much improved Sims for the 212/412 at the FSI facility at the Bell factory.....I only experienced Sim sickness one time....and have never had any problem with motion sickness ever even in very rough water on boats and ships.
The lag between the visual and the Sim is one factor but the most important cause is the way the Sim System works to generate felt forces due to pitch and roll.
In the real aircraft the airframe moves relative to the horizon and once in a stabilized turn or change in pitch attitude that results in a constant relative angle....no other "motion" is felt.
In the Sim....once the "Box" moves per the flight control input....say for example a thirty degree bank.....the "Box" leans over.....and then despite the instruments showing that angle of bank....the Box begins to re-center so it can have a full range of motion available.
Our well tuned hind quarters feel the forces of gravity....and our eyes are seeing something else and the confusion caused by those inputs is what gets us.
The usual victim is the guy not doing much because if you are flying or running the Sim at the Control Panel....each are "busy" and are not free to focus upon the odd sensations.