Every petrol engined, stern drive boat I survey during the sale process, the topic of "Risers and Manifolds" is often a "hot" topic, yes irony intended... I will give you some insight into the reasons and hopefully a better understanding why here.
All petrol sterndrive and inboard engines have risers and manifolds, typically made of cast iron. Risers are part of the exhaust system that "rises" the level of the exhaust outlet so as to keep water out of the engine. They also allow cooling water (sea water) into the exhaust system so as to keep the temperature at an acceptable level.
Typically, in saltwater use, risers, and manifolds can be expected to last around 4-5 years before replacement is required. Engine hours do not predict lifespan, nor is there any reliable way looking at the outside of either to understand the internal condition
The consequences of leaving replacement beyond what is advisable are as follows;
1/ Corrosion (rust scale) blocking the "water jacket" and preventing cooling water flow through the engine and exhaust, resulting in overheating
2/ Corrosion internally allowing seawater to enter the engine, which can result in engine failure
As an owner of a petrol, sterndrive or inboard engine, it is advisable to spend the (relatively small) amount of money on replacement of these items as required to avoid spending a relatively big amount of money when they fail and the consequent damage occurs.
As a buyer, you would be well advised to engage a professional to assess the vessel and tread carefully when told "there's never been a problem" or "we removed them and checked them out a year ago, they were fine"
The below image is taken with a thermal camera of a vessel during a Pre-purchase inspection, shortly before overheat and breakdown. Port side riser reached 169 Degrees C.
Below images shows basic arrangement of a typical riser and manifold that are due for replacement - also note heavy corrosion buildup in cooling water passages creating overheating