The bike shop may have been talking about the durability of the hub. The Nexus 3spd is not meant for extreme forces, but I don't think pulling a trailer would be to harsh. Mounting may be a problem if the mounting point is on the axle. The anti rotation washers may be in the way. But if you can use a mount like the Burley Classic that wedges itself in the rear triangle you should be okay.
I had my Burley with the Classic hitch mounted to my Electra Deluxe w/ 7spd Nexus hub.
http://www.burley.com/products/accessories/index.cfm
Looking at Instep online it doesnt look like they offer options as to the mounting point. You may be able to buy the Burley Hitch to use on the Instep.