I believe the enforceable patents on the Webasto and Eberspacher expired which opened the doors for essentially carbon copies to be made, which saw the advent of these much cheaper Chinese models. There is little within the heaters that could be called cutting edge technology these days, so it was comparatively easy to copy well established designs, Where I share concerns about these heaters is not so much th e burning technology, but the safety devices which you have no way of knowing if they will work properly until it might be too late.
From a slightly different perspective, and this applied to the original western designs also, Forced combustion diesel heaters frequently burn very dirty when they first start up. This can cause soot build up inside the heater, which works against the heater efficiency. Some of the soot and other unburnt by product do burn off as the heater get close to it maximum working temperature, however if a heater is prematurely shut down by a thermostat before it has reached optimum working temperature this can lead up to a build up of adverse residues in th e burner chamber which should be periodically cleaned in accordance with manufacturers instructions. Failure to clean, will impair the heaters efficiency, It can become noisier in operation, and its exhaust will become smellier and smokier.
For example using a 5kW heater where you only need a 2kW one could be come problematical.
It is perfectly legal if you can find a red diesel seller, I must be kept in a separate tank that cannot be used for motive power. THe red dye used in the dutyfree fuel is very pervasive, and traces can be detected at very low contamination rates so never used any red diesel or old containers of red diesel to top up you traction tank There are severe penalties if the you are found to have used red diesel on a road going vehicle.