The cheapest z-wave thermostat currently offered by amazon.de is one of Coqon which is identical in construction to Eurotronic Comet Z. I tested it. I returned it.It fails for me on all levels.
The style of the hardware still is the best of it. Already the display is difficult to read. The letters are not bold enough for the dark edge where a heating valve typically is placed. There is no background light.
When it comes to installation and resetting, you have to operate a switch of microscopic size, that is placed half behind the batteries. You have to search for a tool suitable to operate the switch without breaking it. Touching it three of five seconds make a difference while you are uncertain if you are actually pressing it at all.
If you expect an instructions sheet to guide you this difficult path, you get disappointed again. The letters are also of microscopic size. After reaching for a reading lens, there is even no picture where you find the named switch at all in the battery case.
I tried hard. After hours of exercise I still could not include and exclude the device reliably. Once it is included, it does not cooperate well with Home Assistant. I didn’t get to a point, where I was sure it would work reliably at all.
I really can’t recommend it.