The Dark Angels came in hard, aggressive and honestly kinda dumb.
They lost what 200+ veterans and most of their gear in this assault?
It wasn't kinda of dumb. Except maybe on Vanaheim. On Vanaheim, it was meant to be go in, kill Osmadiel and any other heretic leadership they could only for the Death Guard to get a nat 100 on killing them while the Angels of Vengeance went rogue after Torvald pissed them off. Things did get bad for a bit which is why Osmadiel took a chance on releasing the Death Guard. Once the Angel of Redemption Grand Master was killed, it should have been an orderly retreat where the Dark Angels and Angels of Vengeance got out with light to moderate losses, but the latter got stubborn and decided to fight to the last.
On Midgardia, everything went wrong for them. You used a secret super-death laser to blow up one of their warships, bogged down the ground forces with the prepared Storm Guard. And you hit them with boarding actions, which is your speciality and even then, the 1st Company involved was good enough to repel you the first time. If you had been a normal Space Marine Chapter without secret DAoT tech and a ridiculously well-equipped IG/PDF force, including standard issue Carapace Armour, they would have likely beaten you and taken your fortress-monastery.
At Mjolnir, the Unforgiven did win. Yes, they took a bad hit when the strike cruiser carrying a 1st Company was destroyed, but half of the defending Astartes died in the process. They did successfully seize the fortress-monastery and it was only a self-destruct that denied it to them and took out their remaining ground forces, which were still fairly intact at that point.
So the Unforgiven got two losses due to unexpected factors that they couldn't have reasonably predicted and they got one pyrrhic victory where the enemy was still hurt was then they were.
@Osha What's your take on loyalist chapter legion makeup in 40k?
About 50% Ultramarine and there are no official Salamander or Space Wolf successors. Beyond that, there are a few dozen Successor Chapters each of the other legions, leaning towards more Imperial Fist and Blood Angel successors. The rest of the Chapters are unknown which basically means they don't know. It could be a Loyalist Primarch, a Traitor Primarch or Chimeric, but the records of that don't exist anymore if they ever did or nobody knows wheres said records.