If you are a gringo, any and all accidents are "serious". Are they commonplace? With the number of motos on the road, and the fact that you will be at fault even if they broadside you, yes they are commonplace.
Ever hear of anyother country where you are advised to purchase "casa de conductor" coverage on your policy so that you will be held in a "hotel" rather that jail until the accident is sorted out???
In the event of an accident, you can expect the "victim", his lawyer and the investigating police will have to be compensated.
As far as renting from a non-US based company, again, all will be fine until an accident occurs. The insurance company will take 30 seconds to run your license and deny coverage if it comes back suspended.