Of course the estimates vary based on assumptions, but under some circumstances it could be very valuable.
That’s the more important set of assumptions, in my opinion, because under the current conditions I see assumability as a form of insurance. You have to pay a little more (via the higher costs to FHA loans), but IF rates do go up a whole lot, that money will pay itself back many times over.
When I consider buying, the biggest risk I see is the high probability of much higher rates in the years to come. By getting an FHA loan, I can mitigate this risk to a pretty good degree. So I see the assumability as an insurance policy that protects me to some degree from rising rates, and since I think a steep rate rise is likely, it is very much worth the higher cost for the FHA loan.
