I think the elven benefit there is strategic, while the orcs enter the forest and are not native, having no special skills there, the elves can hide and pick their battles. An overwhelming number of elves can strike the smallest group of orcs by surprise during the daytime, reducing the orcish army's numbers slowly. Perhaps the orcs decide to burn down the forest, that's fine, but it's not going to happen overnight. Most forests are not like the hyper-matchbox managed forests in California.
Secondly, orcs aren't known for their missile skills. They're face-to-face smashers.
Take the elves out of the forest and make the battle at night in melee range, sure I can see the orcs having the advantage.
And of course the orcs have a more militant culture and there are a whole lot more of them. I don't see why an orcish army vs. an elven army wouldn't outnumber the elves by 10:1. The elves need that stealth to pick their battles. If they can't the orcs simply demolish them.
So it turns into this: orcs run wild everywhere except where someone else has an exceptional advantage AND is militarized enough to take advantage of it.
Orcs attacking a mountain valley or pass defended by dwarves - I don't see the orcs winning even with 10:1 numbers simply because the dwarven structures will funnel them into fights where it's 1 orc to 4 dwarves and the dwarves are behind fortifications. Much less all the orcs slain by traps, rockfalls, etc. But stick 100 dwarves out on an unfamiliar plain with 1000 orcs (again, orcs are far more fecund than dwarves), and the dwarves get slaughtered.
Orcs vs. humans I'd say would probably be 3 humans per 10 orcs. But the humans again need terrain or some other exceptional advantage, because roughly 3:1 numbers favoring the orcs means the humans are dead meat.
It's really about what numbers you can bring (that is, what army can your civilization produce and support in a war theater) and what force multipliers you can include. Orcs excel at fielding a huge army but generally employ few force multipliers.