[@Savato]
The orks aren't that powerful. Every ork is basically half a space marine. While they are one of the most powerful factions in w40k, they spend most of their time infighting. Most likely an ork invasion would be defeated by a tiny bit of subtlety, rather than any real army-on-army action. 

The tyranid can't be defeated by army battles. They just can't. Any time you destroy their troops, they just make more. Literally infinite armies, because they recycle the biomatter of the dead into new soldiers. The only way to defeat the tyranid is to destroy the source, to send an elite team of warriors to destroy either the hive fleet (with the norn queen), or the current hive tyrant, in the case of a smaller army. Once it's production and leadership is down, the hive fleet collapses. Head of the snake. Now if that doesn't sound like a cool story to play through to you, then I disagree with your tastes.