Its easily the most mild of any opioid withdrawal and I've experienced many different forms of varying severity from many different opioids. I've experienced withdrawal (on in some cases multiple occasions) from: heroin, methadone, oxycodone, morphine, tramadol, o-desmethyltramadol, opium (via poppy pod tea), tianeptine, loperamide, 2-methyl-AP237 and kratom. I am probably forgetting a few.
Even at decent size doses (35 grams a day), cold turkey is possible with comparatively mild symptoms when compared to other opioids (even in equianalgesic doses).
For me the symptoms have never risen above joint pain for up to 5 days, a running nose and yawning for 3 days, insomnia for up to 6 days, loss of appetite for up to 5 days, and mild depression and boredom (this is the longest lasting symptom generally speaking for any type of opioid withdrawal). Aside from boredom/depression, the main withdrawal seems to over in 5-7 days.
The symptom somewhat unique to kratom withdrawal is a particular form of restlessness. This likely stems from kratoms calcium channel blocking properties. Restlessness is featured in the withdrawal from other types of opioids, but kratom withdrawal restlessness feels a bit different. Kratom is likely a NMDA-antagonist as well to some degree so this may have a role in the unique kind of depression it produces upon withdrawal.
Kratom has a complicated mechanism of action so you are withdrawing on multiple fronts, but compared to other opioids its far more mild. I've never really exceeded 35g a day (not because I didn't want to -- the dizziness/nausea is just too strong) so I suspect that those that had issues are taking much higher doses than that.
If were to compare it to the withdrawal syndrome of another opioid I'd say tramadol, which is also a dirty opioid who's withdrawal syndrome is less severe than others.