Lunar Year Mandarin hits like a truck, his 3-hit combo is pretty nuts. But the holiday bonus makes him worse than any of the other cosmic terminal bosses, which I'm sure is intentional.
I'm not overstating the value of defense, though. The Captain America, Thing and Colossus I run with generally facetank Power Cosmic right now in cosmic terminals and don't need to move around typically unless both Power Cosmics happen to overlap. I think only the Cap'n has LL but they all have 25% HP on medkit, because the 25% HP medkit cores are just more powerful with how high health scales. I can't really do it on Wolverine, but he's like 6th lowest overall mitigation in the game too, and I still solo clear cosmics pretty handily.
Getting 10% more raw mitigation is a big deal. If you're at 40% mitigation and you go to 50% mitigation, you are taking almost 17% less damage. This multiplies the effect of both your dodge and your health (the latter being the big part due to current itemization). Dodge rating is definitely harder to gear for, which is part of why Fandral is so far ahead of everything else. It's even better for Dodge heroes, though, since you get around the dodge DR completely.
Power Cosmic does suck, for the record, and I expect it will get tuned down very soon because not only is it a lot of damage it is way too relevant for melee compared to ranged heroes. There is also still a lot of survivability discrepancy between melee heroes; Ms. Marvel is absolute faceroll even in cosmics whereas you have to work a lot harder on Wolverine or Nightcrawler, and both ends need to get reigned in a little. It doesn't change that gearing defensively makes a huge difference for melee in cosmics, and honestly I don't think it's a coincidence that most of the people saying otherwise are the same ones getting their shit pushed in so hard. Dodge heroes like Wolverine, Nightcrawler, Black Panther I would try for 10k defense, 35% dodge and 20k-ish health before being able to solo comfortably.
Cosmic terminals are basically the limit of how far they are willing to scale content up in base difficulty; this is MP10 for Marvel Heroes. Even raids aren't planned to be as much of a gear check. It seems pretty normal to me that melee are more pressured to gear for more survivability in the highest tier of content, as I haven't really ever played an ARPG that has worked differently. Both reds and greens need to be adjusted upward some, mainly reds. There definitely is too much of a gap between the two, as I think a lot of players assume if they can handle reds then they should just move on to cosmics, when the requirements are not a small jump at all.