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.