Well, I have the three worst dark events to put a stop to this time...

[sick duck]

Alien infestation, left behind, and alien cypher. All three of them suck ass and I can't stop them all...

[fewerjungle]

Alien cypher is bad if you need to expand. Otherwise you can do other scans and build relays.

Left behind doesn't apply to all covert ops I think, but usually the chosen hunts will have them. Sometimes an extra soldier can be used to counter it.

I don't think extra chryssalids from alien infestation burrow. If they do that can be a problem.

It kinda depends on what you're doing on your campaign.