I have extensive experience on a Woodmizer mill. They use a hydraulic tensioner with a glycerine filled hydraulic gauge. One thing which is very important to note is the thermal expansion of the blade.
It is effectively impossible to maintain appropriate blade tension without some sort of readout. Depending on what kind of wood is being cut, (dry oak for example), and the ambient temperature (winter), very soon after initiating a cut, tension can fall off 20%. It will take a very experienced sawyer to adjust appropriately by feel.
Also, bandsaw blades need to be untensioned when not in use, and are very susceptible to dullness, as mentioned, dirt in the bark, or general wear. It is never worth it to push a blade past the keen cutting stage.
Also, tooth setting is critical to proper performance. If not set perfectly, and symmetrically, (and sharp), the blade can tend to veer up, or down, or cut funny around knots. Blades must be set after grinding to within thousands of an inch offset both sides in order to work optimally.