Some counter points
They're genetically driven to fight, yes. They're also an inherently migratory species. You can't afford to simply leave them to it, because sooner or later, the constant fighting will see a Big Mek with the resources to build a Kroozer. Or the spirit of the Waaagh will kick in, and they'll feel compelled to find someone else to beat up.
And Orks very much can bash together a starship. It's what they do. Heck, half the time a Mek is just building
something - and won't know what it is until it's finished.
Orks are self sufficient species. When they arrive on a planet, it's 'orkiformed' as their spores start producing Squigs. And from Squigs, all things are derived. Food, fuel, ammo, beer, weapons. All sorts.
Any offensive mounted is overwhelming. You're talking hundreds if not thousands of Boyz who's only wish is to get into a fight with the enemy. They're resilient, and they're motivated. They literally live for battle, and proving how 'ard they are.
The Orks are nutters, with a unique society. Wherever they go, they thrive. They do stuff instinctually. Their politics is wonderfully simple. If you can clobber them, you can boss them around. If not, they're the Boss. That's a pretty major strength, as every Orkoid (including Grots and Snots) just accept that's the way things are, and crack on with their short, terror filled, miserable lives.