It is unique among other types of personal injury insurance claims. There is not something that is so unique that a personal injury lawyer wouldn't know about them.
Ok, perhaps adjusterjack meant to say it's only unique to people who are not lawyers.
My POV, personal injury claims are just as unique as UIM claims since I am not familiar with either of them.
From what I can see, UIM claims have a longer statue of limitations, I was
told verbally 10 years by Allstate but I have yet to confirm that, in disputes they don't typically allow lawsuits, but there are exceptions such as claims that exceed 75,000, Allstate pays for the costs of arbitration, whatever the primary insurance paid out subtracts from the total claim. Typically UIM claims are settled after medical treatment has been completed, with the caveat that there is still a statue of limitations involved.
There are some gotchas such as the at fault insurance being higher than the UIM insurance, so if you collect 100K (policy limit) from the at fault party you can't collect another $25K (policy limit) from your UIM insurance, not sure if that is universal or a state by state situation.
There are a lot of other rules that are different, those are the ones I got so far, and many of the rules only apply to uninsured motorist not underinsured.