Manker is quite well-known brand among flashlight enthusiasts, but they don’t have a good opinion. The main reason is the lack of manufacturer support and problems with the warranty. Although the flashlights are imho a very good quality, problems often do arise.
Another reason is their (old?) UI. Reproduced in many flashlights and almost always criticized. Nobody likes long click for on/off. The new one looks much better.
See this thread where people complain about Manker. I have a few of their flashlights, but I don’t use them often, so I won’t comment on that. Although I had no problems with them.