Which talents are you using and what play-style are you employing? Using the right talents makes a whole lot of difference like with everything else. And with surviveability with large numbers of mobs, a good rule of thumb is to a) try to hit as many as possible, b) eliminate the weakest target(s), c) don’t just stand around in one place, and d) don’t massively overpull.
I am pretty much guaranteed to take out 5-6 mobs easily without a dent in my health. 8-10 I have to be more careful but I can make it. 12 is pushing it, but I can make it if I pop blur and use Chaos Nova before resuming the normal rotation. Anything past that is a crapshoot.
Heck, I can actually solo many world bosses easily with barely a dent in my health as long as it doesn’t involve frequent long stuns and adds with immobilizing abilities.
Havoc is only squishy if you get stay in one place and get massively overwhelmed. The key to mastering DH at least is to be very proactive in how you move.
To use an example, the other day I was doing a Legion world quest involving a world raid boss thinking it would be easy pickings like every other world boss (Lady Si’vash). I was dead wrong because she had a LOT of long stuns and her adds constantly rooted me down. And then I kept getting caught her her tidal wave attack because I did not know what I was doing. Combine that with her insanely high health, I barely did much damage even with another overleveled player who also underestimated the boss. Honestly, that fight would have been impossible to solo even as a 120 toon with an ilvl above 320.
I was lucky the Alliance boomie was there and we were able to figure things out without the ability to communicate ideas. Even still, we only beat her on our third try. It was on the second try, when I tested to see if Vengeance does better (it did not) that I realized as long as I dodged the tidal wave and avoided getting immobilized, she couldn’t do much to my health. So once I did that on my Havoc (and the boomie tanked the adds away from me while I kited Si’vash around well away from them), I easily whittled her down within about 10 minutes.
This is actually a consistent pattern I noticed when playing a DH. I only am squishy if I get swarmed. On the other hand, if I kite and utilize my CDs and pop an eyebeam, I typically have a lot of surplus soul fragments at the end.
So yeah, playstyle matters a lot on DH. I rarely die on him and usually when I do die it’s either because my game lagged and screwed me over or I made a serious miscalculation on how many and which mobs to pull.
Edit: Also, I should mention that knowing when to make a quick retreat helps too. If you’re getting overwhelmed and your eyebeams are on cooldown, using vengeful retreat and then fel rushing away usually gives you enough time and distance for the cooldown to end so you an instantly pop the eyebeams on the first few to catch up to you. The eyebeams recover part of your hp and the soul fragments from the mobs you kill cover the rest. This method has saved my life more than once.