Yes, some units need to be carefully adjusted -- but others have a spring tensioner and require almost nothing to work very well. The best one I have used is a
Husqvarna. Another good one with a spring tensioner is the Gilmore 20". I wish Husqvarna made a wider model.
The Scott Classic is a screw adjusted unit, and it works better on thick/coarse grass than the Gilmore, but ya gotta' work at it a bit.