Holds up a 34" ultrawide no problem, I would rate it 5 but it does slowly tilt forward over time - the tilt joint just needs a lock nut for heavier monitors
John__wI put the LG 34UM88C on it, also about 18 pounds. The reason I bought this unit over ones on amazon was the extra weight capacity of the base and gas-spring.
The arm itself handles the weight no problem. It maintains height perfectly.
You do have to keep the tilt joint secured very tight though to stop it from tilting down and I probably wouldn't want to put any more heavy of a monitor on the stand for that reason.