I think the biggest liability arises when you install the signs. A hanging sign, above a sidewalk, a stiff breeze, sharp edges, is a combination with a potential for disaster.
A truly outrageous sign that commands attention could be another possible liability. For instance; a sign depicting a lovely woman ( or man) about to peel their clothes off may cause motorists to be distracted leading to an accident.
There is also the issue of politically inncorrect signage. If a sign of your making can be construed as maligning any minority, religion, identifiable group, you, as the sign maker, could also be embroiled in a lawsuit even though you created the sign according to the specs of your customer. Lawyers use a "shotgun" approach and will sue against any party involved in the alleged "infraction".
You should have WCB coverage. If you fall off a ladder and break your hip..you'll be real happy you have that.
Ken