You're right. But there is a deficit of good, hard data on many of the treatments used for cluster headaches.
In additions to the case review you cited, there we have informal reports from clusterheads posting their experiences here and elsewhere over a period of close to ten years.
Data from Internet reports is hardly scientific, but it was all we had before Sewell and Halperns' case review, and the results of the Internet reports are not inconsistent with that study.
In a collection of reports from 378 people, 240 or 63.5% reported significantly positive results, 48 or 12.7 percent reported little or no improvement or some other outcome that made the treatment unacceptable. 90 reports or 23.8 percent are indeterminate, meaning people never got around to reporting how the treatment worked out, they weren't sure if the treatment worked, or it couldn't be determined what happened for whatever reason.
If the indeterminates are dropped from the count, the results are 83.3 percent positive and 16.7 percent negative.
So there are plenty of reports of the indole hallucinogens not working, and I'm sure there are cases where it did not work but there are no reports, just as there are many un-reported cases where it was successful.
I'd like to show you reports of unsuccessful use, but there is neither room here, nor time before I have to go to work. In many cases, people were using steroids or triptans that apparently interfere with indole's effectiveness. In some cases, it's possible the substance had no potency, or the doses were too small or too few, or people didn't tolerate the side effects well, or were worried about legalities and stopped the treatment, or any number of other reasons. In some cases, no matter what was tried, it just didn't seem to work.
As to variations in potency, that is a problem with natural substances, but it can be minimized. One way to judge is by describing the level of psychoactive effects; with seeds, this can be a problem because the psychoactive effects can be slight or imperceptible.
Some of the early reports can be found on Multimedia File Viewing and Clickable Links are available for Registered Members only!! You need to

or

- look for "Message Board Archives" toward the bottom of the right-hand menu.
Others can be found by do a search on this site's message board archives.
This was going to be a short answer, but sorry, I guess it didn't turn out that way.
-tommyD