I ordered the version with the ESP SER2NET bridge. All MCUs came already flashed with a usable firmware. So it was just a matter of plugging it into a USB power supply and connect it to zigbee2mqtt via TCP. It is running for a couple of weeks right now without any issues. I am happy. ;-)
This ZigBee coordinator and router delivers on its promises. The range is far better than I expected, and I am now able to communicate with devices I couldn't reach before (even with a CC2531 with external antenna).
Flashing via USB works exactly as expected, and it was really fast as well!
The shipping was okay, but since I ordered without tracking, it took a few weeks to arrive. I'm happy I waited though, because this product is worth all the money.
This coordinator/router combo cannot be beaten by anything else at this price point. Can only recommend this to everyone!