With the Orks you're looking at a mixed group of enemy types at any given point so its about evaluating who is providing the greater threat on a moment-to-moment basis. Individually an Ork may not be very threatening to a Space Marine but as a group they can definitely be a threat so you have to always keep your eyes open on who is showing up and what is coming next.
With Chaos the encounters will be more tactical since they have higher-powered range strength, enemy types like the Bloodletter, which is a demonic enemy, has a very powerful melee attack thanks to a giant sword but can also phase through reality. So they don't just run around, they phase from point to point which makes them very hard to target with ranged weapons. Their whole idea is to get as close to you as possible to deal a lot of damage, they fight together in packs of three or four and are kind of like Raptors in that one will distract you while another tries to flank, so they're much more devious in that way.
We've also got an enemy called the Tainted Psyker, which is a magic user that can attack with its own range attack and spawn units up close to you. When you're fighting these guys you never really know exactly where the enemies are going to come from so you have to have situational awareness of what is happening on the battlefield. In combination with the Bloodletters and Psykers the enemies will close the distance and force you into close combat situations.
