The first reason concerns the safety hysteria. The second reason has to do with glass (ceramic) flat-top stoves (if you are interested.)
The first reason: certainly, there are valid home canning safety concerns for any type of canner - if you do not follow proper canning techniques, or use the proper canner for different types of foods, then you can run into a problem. This steam canner is perfectly safe for acid foods (like pickling, etc.) as long as you following the directions. Steam is a minimum of 212 degrees, and when you follow the directions by making sure you have a continuous 8" column of steam coming from the vent holes before you start your timing, there should be no concern about "cold spots." If you start timing before this, then just like a water bath canner, you may underprocess by mistake. So, follow the easy instructions. The USDA's position is that there is a LACK of evidence for them to say steam canners are safe - it just means no one (or not enough scientists) have done research to make a definitive statement (it doesn't mean they are not safe). And this is only partly true - research has been done, and this type of steam canner is safe, as long as you follow proper canning techniques, follow the directions, and process only acid foods with it. For more information on the position of safety, the research that has been done (and the opinion of one dissenter who has NOT done any research) [see comment section for link]. If you have seen research to the contrary, please cite it so that the rest of us can look it up, too. It's also worth noting there is only one safety recall for any type of steam canner - and it was a plastic one (Micro Dome) that was to be used in the microwave - furthermore, the safety issue was about the device itself, not potential food spoilage.
The second reason: I have a glass (ceramic) flat-top stove, for which this canner is not recommended. But it's possible if you have patience and the right type of glass flat-top stove. My stove is a newer model Kitchen-Aid in which the entire glass top is re-inforced to endure the extra heat from canners (if the glass around the burner areas are not reinforced, it could crack under the extra heat that a canner would create due to the over-size surface area of the pan bottom. The radius of this canner is a couple of inches larger than my largest burner. It just takes a very long time to bring it up to temperature. On high, it can take anywhere from 20 - 40 minutes to bring the water to boiling (it takes an hour in my larger water bath canner). Once the jars are placed, it can take another 20-30 minutes to bring it up to the proper temp (8" column of continuous steam billowing out of the steam holes) before you can start the processing time. So, if you have a newer, reinforced glass flat-top stove, you may still be able to use this (and other canners).
I have a very large water-bath canner, but wanted something a little smaller and something that uses much less water. It's easy to use - just follow the directions, and don't forget to wear hand/arm protection - that steam is extremely hot.