The sidecar turns the machine into a rig, not a bike - nothing at all you know about riding a bike counts. It is more stable, and you don't need to put your feet down, but after that everything is different.
You don't ride a rig, you drive it - steering is something you need muscles for, as you have to turn the handlebars in the direction you want to go. Speed is also a different sensation with a rig - it is a lob sided three wheel vehicle, and as such, behaves oddly. To maintain rapid progress round corners, you have to hang off the side of the seat, in the direction you wish to take, or you will not make the corner - In left hand bike, right hand sidecar situations - US setup, the left handers tend to lighten the rear of the bike, making a nose dive very possible, and the right handers lighten the sidecar wheel, making it highly probable to lift.
I try very hard not to lift the rear of the bike, that can hurt, but the sidecar wheel is often up in the air - known affectionately as flying the chair.
I prefer the carrying capacity of the rig - I can get a day's purchases at the supermarket into the trunk, or round my Lady's legs - try that on a two wheeler.
The techniques required for safely controlling a rig are totally different to those needed to control a solo motorbike, so could be construed as more difficult. The strength requirement is very real - the faster you go, the more difficult it is to turn corners. There are skills that need learning but when learned, the rig I have is way more fun than anything on two wheels. (mine is two wheel drive, and will go anywhere a jeep will.)
So, your dealer is right to say it is more difficult, but the truth is somewhat more involved than just that.