And when the map is mirrored and not rotated symetry then their statement already becomes invalid as you would gain diffirent advantages around corners anyway.They go insane over the tiny differences.
But yes, there is a point of logic on why they go mad about it and i think its at least worth to explain why its a problem:
When it affects movement time its a bad thing as then it matters because the object would be hindering that team for capping (if you get on the cp 1 second to late it requires you extra time to counter that, time in which the enemy could be closing in). And even though they dont fall in the path it still matters for balance as instead of going for the cp, you might want to take the mid area. All the movement actions should cost the same time.
And when blocking visibility both should (it doesnt have to be an exact mirror on that part, but the gain/loss from being (un)able to see through it must be equal for both teams. After all, if you hold the point and can see the enemy medic earlier as the forklift can be seen though that enemy medic will be spotted earlier and probably also have to use his uber earlier. Yet at the closed boxes he doesnt have to.
And the crate vs forklift problem affects both of these. So thats something that should be fixed when aiming for comp.