It's also not that common that sponsorship undertakings are not approved, from my observations. If the sponsor candidate is found to be ineligible to sponsor, CPC-Mississauga would reject the sponsorship portion and I believe that most people check off, "withdraw your sponsorship application. All processing fees less $75 will be refunded," in IMM1344. I know I did. In the event they check off that they want to continue with the second portion of the processing, the sponsorship guide states that, "The visa office will likely refuse the application for permanent residence and inform you in writing of your right to appeal." Again, the combination of sponsor refusals being low and checking off to continue with the application in the event of a refusal being low (in my humble opinion) means being refused by the VO because of the sponsor not too common.
It doesn't matter where you come from,the most common reasons for refusal include the sponsor is in receipt of social assistance, is bankrupt or is in default of a previous sponsorship agreement; or Immigration Canada believes that the marriage may be one of convenience.