Grant was a great General.
His Vicksburg campaign is still being studied at West Point. He is also the only Union General to force the surrender of 3 Confederate armies.
The US Civil war was the prototype for Modern War. What seemed so shocking to his Democrats poltical foes (Both Confederate and in the North) was Grant was the 1st General on either side who understood that the only path to victory was the utter defeat of the South’s will to wage war. The only way to end the war was beating the South until it was on its knees.
So since they could not argue the facts, they engaged in Democrat Politics 101, the smear job. Grant historical legacy, only now being corrected by modern historians, was largely a create of Lincoln’s, Democrat political foes, and sore losers Southern Generals.
He was. I tend to think Lee was the greatest commander of the war because he did so much with so little. However, one reason Lee had that one great year of victories was that all the Union commanders before Grant (and Meade) would retreat even though they outnumbered the Confederates in men and materiel.