I have never seen this in over 5 1/2 years of playing. Terrain has never made it impossible for units to move. For either my units or the enemy units. Please attach a screen shot of this extraordinary phenomenon.
I know I'm late to this, but I've personally experienced two occasions did the terrain mess with Units regarding their movement.
One was when a Rogue of mine was attacked and transformed into a Champion while standing on some rocks. As we all know, those horseback Units cannot stand on them; thankfully I was able to move it out since there was clear terrain around, but if that Champion had been surrounded entirely by rocks, who knows what would've happened.
The second time was during a battle with my PE Units and the enemy's Indy Units (can't remember if it was on the C-Map or PvP, but probably the former). Basically, a massive trench (well, it was more than one, but you get the idea) split the field in two parts. Trenches, just like the rocks, don't allow horseback Units to walk over them, and the AI made those enemy Lancers stay put throughout the entire battle. Literal sitting ducks for my Artillery.