I played my first two matches with her ever on stream. My Khabarovsk was still locked in a Battle, so I figured I'd play it as a variant on my Khaba playstyle with armor and range. My experiences were... awkward to say the least.
She handles like a pig if you don't have the Engine Boost on. When it's working, she feels like a normal Destroyer, but when it's down she's paaaaainfully sluggish. Folks that enjoy the ship have taken to using the Slot 2 Engine Boost Upgrade you can get for Coal in the Armory. Anything to add time to make her not feel and play like a Hippo in an ice-skating rink.
I also found her shells to be floaty to the point of frustration out to the 14-15km I like to operate at, so that playstyle ended up being a wash.
My chat told me to instead use Double Concealment (Slot 5 and Commander) to drop her detection to 7.5km. In that way, you have only 13km base range and won't be able to range farm as cleanly, but you have the ability to hunt enemy Destroyers. Once you are detected, you can Radar and force a fight. While she might not have the DPM of your Friesland, she has tankiness in spades. Huge healthpool, solid armor that shatters non-IFHE DD HE shells, and heals make it so that she can force a fight and always end up the winner of the trade in the end.
I ended up reverting my Slot 2 back to Radar and away from the longer Engine Boost. My decision was that if I'm using her as DD trading/harass, that I wanted the longest window possible to force the situation in. The engine boost natively lasts longer than the 40 second Radar Window (Slot 2 and +10% Commander Skill), so I have the tools to make my threat windows matter as best as possible.
This is what I'm currently toying with, but I'm still new to her and might tweak her further.
As for the "Ragnar Rage", yeah... other ships really HAVE to help work on the Ragnar. DDs can be massively out-traded by her, so if the bigger ships don't help out they are just actively cede'ing ground to the enemy. A Ragnar that feels free to force situations can be devastating to your frontline's capabilities.