I have been getting challenges that cannot be won. Kill X many of a creature that is not is not spawned, not will spawn during the challenge, or even on that mission at all. Gather 1000 shards after the whole team cleaned out the area to pay for turrets 2 objectives ago.
Had the same issue with some challenges, especially the “kill the X number of enemies in 45 seconds” ones. In some missions, no enemies have even spawned yet and, by the time they do, you only have about 10 seconds to complete the challenge, making it impossible.
The most that I have received from the challenges are blue loot packs or 25 credits. While it’d be nice to have obtainable challenges, the loss isn’t substantial enough for me.
Worst place for challenges is the Archive though. Clear out all shards in the first half. Double back and get a 1000 shard challenge.