Back in the day when I was working on another RTS of the era (Dark Reign 2), pathfinding was the bane of our existence. We weren't able to dial the pathfinding in because we'd pivoted late in the development cycle from the more innovative but complex 'walk anywhere on a 3d terrain' design to the ship-it-quickly traditional grid unit placement of 2d RTS'. This was the era of 2d RTS' transitioning to 3d, so everything was new and a bit harder than anticipated.

Just that one thing of having sub-par pathfinding made the game far worse, I'd guess at least a full point off a 10-point game rating. I believe we had to slow the whole pace of the game down because the player had to babysit units as they moved, which made the game far different from its extremely fast-paced predecessor and which the players expected from a sequel. I played DR2 again recently and it doesn't hold up almost solely due to pathfinding and the pacing that results.