I never had a problem negotiating with an insurance adjuster and always had a positive outcome
The insurance companies are pushing for states to forbid anyone to negotiate with insurance claims unless they are a licensed private insurance adjuster in the state where the claim is, or an attorney in that state.
You mentioned you testified in court cases as an expert witness. So that means the policyholder had to foot the bill for an attorney and your fee as the expert witness to try and recover their costs for the claim that was originally underpaid otherwise you would not have been in court at all.
So the end result if they recovered additional money from the insurance company was they still were shorted because now deduct the legal fees, court fees, time away from work dealing with the litigation of the case, etc etc.
Watch the video I posted of the two lawyers, one man grey hair represents insurance companies, the other lawyer in the bow tie represents policyholders.
They are at a seminar for roofing industry contractors but the tactics are still the same with all claims.
Other videos by that same group talk about how insurance companies refuse to pay for starter shingles and ridge cap shingles which are required to do the roof correctly. They flat out deny the costs and will not speak to the roofer about it only the homeowner or a private adjuster or lawyer.
Private adjusters charge a fee of 10-20% of the total claim amount so the policyholder is short that amount for the work and even more if a lawyer is involved.
The game is rigged.