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The later Vee-Dubs and Porsches went to a fail-safe system for the most common failure mode of the exhaust heat exchangers. They made the exhaust pipes out of (a low grade of) stainless steel, and the heat exchanger "wrapping" out of mild steel. The wrapping would rust out first--which would be visible from the outside, and would have noticeable effects on the heat delivered by the system. The exhaust could then be replaced, in theory before any leaks could develop from the exhaust pipe into the wrapping.
I will second that there is no extra backpressure created by an exhaust heater system. The heat exchanger part simply is wrapped around standard exhaust pipes, with no changes to the inside of the pipes at all.
The heat exchanger system in my 914 works so well that I have to open the windows any time I have it on! Driving on a nice cold evening (~35-40F) with the top off, I was quite comfy in my jeans, t-shirt, and leather jacket. Though my toes got very uncomfortably hot...
I think that one of the reasons that the liquid coolant is used to heat the cabin air is that it is easier in most cars. The radiator core is pretty compact, and only needs two (relatively small) fluid lines going to it. Exhaust heat exchangers need to move the heat from the exhaust system into the cabin, which can require significantly larger air hoses.
For vehicles with large interior spaces (e.g., Microbuses, minivans, SUVs, large sedans) you can also put heater cores here and there and anywhere and use a fan to circulate cabin air through them. For an exhaust-heat system, you would have to run more large air hoses to where you wanted the warm air to be, and you would have to push that much more air across the heat exchanger. That gets impractical pretty quickly, I would think. I also think that was one of the primary problems with the old Buses, they were trying to heat up a larger volume of air than they could reasonably heat up. Vehicles with smaller interior spaces get quite well warmed, in contrast.
Finally, there is the perception that people have of exhaust heat exchangers as "unreliable, smelly, and prone to kill you". That mostly hasn't been true for a while (and the true parts can be addressed with some redesigning) but the perception exists, and will make it difficult for anyone to sell such a system. (Check the US consumer's reaction to diesel cars over the last 20 years, most opinions were poisoned by the loud stinky underperforming Chrysler diesels and big Benz diesels from the 80s. Even to this day, when new diesels are clean and quiet and torquey as heck, there's still a resistance to them.)
-soD
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