I regularly remove chimneys and sidewall vent gas appliances at my rental rehabs for exactly the reasons you have mentioned - a major source of roof leaks is eliminated, I no longer have to worry about maintaining the masonry, and I pick up rentable floor space.
It is true, tough, that CAT IV appliances are more expensive and require more service, at one project done in 2005 I have five CAT IV furnaces, THREE have required service this winter (two pressure sensors and a binding inducer fan).
OTOH, the 5 Takagi tankless water heaters - more complicated devices - are so far working perfectly.
I wonder where I can buy Japanese furnaces.
