I generally only check these boards once a year, but I thought this was an interesting time to chime in.
I didn't start having clusters until about 2015, and I left the Army in 2008. I also didn't file for disability until last year (January 2024). I did a lot of research before filing my claim, and from everything I could tell getting clusters linked/approved was almost impossible (I found lots of appeals denial letters which are public record).
I had an awesome C&P examiner, and after discussing everything I believe she wrote up a very convincing argument to link them, although I don't know exactly what she wrote. When I filed, I believe I had it as secondary to PTSD. But I did also mention that I had a migraine (my first and only) at the very end of my service. I don't remember the exact details, except that I know we had already moved off base because it started on the drive home from base right before I started terminal leave. In retrospect, it's very possible it was my first cluster attack, although it was also a one off event. All that to say, I never saw anyone in the military for it.
Additionally, a big thing with any headache for VA is it needs to be debilitating, and frequent. This is where I think a lot get denied, because episodic clusters won't get rated (almost everything uses migraines as a template). Luckily (or unluckily?) for me, when I was filing, I started seeing a therapist for the PTSD which caused me to have the longest cluster period I've had, lasting 5 months or so. The examiner decided that even though 2015 until 2023 had been episodic, that I was, at the time of the examination, chronic. I also made it clear that cluster headaches prevented me from having a normal work schedule, because once they come on that's it.
In the end, I was rated at 30% for them, which I think is a miracle.