You still have to qualify with the seller's lender under the same stress tested standard CMHC applies to any other application, so an assumable mortgage does not let you skip the underwriting the lender requires.
The remaining balance almost never matches what you need to borrow, so plan on covering the gap between the purchase price and the assumed balance with cash or a second charge, which changes the actual saving once you run the full numbers.
Ask early whether a specific listing's mortgage is in fact assumable, since it is not disclosed by default and arranging the assumption takes lender time your possession date will not wait for. Get your hand-reviewed valuation from Joel Dyck.