This is probably the most annoying aspect of StarCraft 1 / BroodWar (1998), considering the game still has a very strong player base and is very enjoyable to watch (even if the clunky mechanics are too much to bear for me to play it).
> You'd send your army to a location, and if there was an obstacle in the way (such as a forest), they'd awkwardly path around the edges of it, one tile at a time (causing massive congestion, because the units at the front would obstruct the ones behind). They'd arrive at their destination one by one, and if the enemy was waiting, they'd get slaughtered.
In SC1, they wouldn't even arrive on location half the time, because the pathfinding is so stupid they'd get stuck trying to walk up a wall. It's so infamously bad it's still being mocked: https://youtu.be/mCEZ2hIcUW0?t=134 - still, a great game!
In SC2 (2010), the pathfinding has been massively improved. You can give orders to hundreds of units and they will all figure out how to navigate complex terrain. They will still clump up at chokepoints or occasionally get a bit stupid when trying to path around other units that are engaged in a fight but this is more easily fixable with even a tiny bit of micro.
However SC2 makes no attempt to make units maintain formation. So if you're moving your army across the map, still best to keep an eye on it, and group up / pre-split / set up before attacking. It's a bit less of an APM game, and more of an SPM (screens per minute) game.