Most crematoriums include elaborate air filtration systems. These are far and away better scrubber systems than you would find at a chemical refinery but the media lifespan is very low, and the maintenance cost of these devices is nontrivial. The cost is worth it however...the stench of burning bodies is to be avoided in any way considering most crematoriums are adjacent to fairly residential areas. Most furnaces have a 'hard stop' at a certain number of operations to prevent any possibility of overwhelming the filters. There is also a specific mass limit to each cremation chamber, not unlike a normalizing furnace or heat treatment furnace.

Crematoriums hate two things as far as i can gather: medical implants and the obese. If enough furnace matter includes exotic plastics and metals, some fairly dangerous conditions can occur. certain plastics and magnesium parts for example can trigger a fire inside the furnace that can easily damage or destroy it. Crematorium furnaces are equipped with, in my opinion, a very limited amount of thermal refractory material compared to other industrial furnaces.

Finally the sad truth about obesity is that you'll pay triple for a cremation. Obesity increases the combustion byproduct and shortens the furnace filter and burner lifespan as it increases the time required to properly cremate the remains. large enough people require multiple furnaces.