Jacob's words over Issachar are the shortest route into the tribe's reputation, and they will not settle into one meaning. He is a strong ass crouching between two burdens, or between two sheepfolds; he saw that rest was good and the land pleasant, and bowed his shoulder to bear, and became a servant under tribute.
Read as rebuke, this is a people who preferred comfort to independence and accepted foreign labour obligations in exchange for being left alone on good soil. Read as description, it is simply a strong agricultural tribe on the best land in the north, doing the work that land requires, with the tribute the price of holding it. Ancient and modern commentators have taken both, and the Hebrew is genuinely open.
The geography supports the second reading and explains the first. Issachar held the Jezreel valley and the slopes under Mount Tabor: the most productive ground in the region, crossed by the trade road from Egypt to Damascus, and therefore the corridor every army in the ancient Near East used. A tribe there would be rich and would be repeatedly obliged to accommodate whoever was marching through.
The tribe is not passive in the record. The Song of Deborah names the princes of Issachar with Deborah and with Barak, going down on foot into the valley, which is the same ground. Tola, one of the minor judges, is of Issachar. And Baasha, who seized the northern throne and held it for over twenty years, came from the tribe, so it produced a dynasty as well as a proverb about patience.