Any reason to not use one? I have a new carbon fork (1.5" tapered carbon steerer) for my gravel bike, and the previous crown race was a bit of a hassle to install. Some online opinions of a split crown race think it might attract more dirt or whatever, but not much else to say.

This would be a factory cut crown race, not a DIY hacksaw job.

Never seen or installed one, have always used solid ring crown races but then again our work doesn't run into too many carbon forks

Here is a video I found on them

Split crown races are easier to install but might not stay put as securely and if allowed to move about (loose headset adjustment as example) might result in wear/damage to the crown's seat. I take the split race thing to be about working around poor dimensional control of the crown seat during the fork's manufacturing. With a variable race ID the crown seat can be off dimension and have less headset adjustment issues.

BTW if I saw someone using a hacksaw to mill down a crown seat I would run in the other direction Andy

It's more about function and terminology.

If it's truly a crown race, meaning the lower bearing balls run on it, than a split race isn't an option.

OTOH if it's merely the centering cone for the lower cartridge bearing, then split is fine, same as it is for the upper bearing.

I picked up a Velo Orange split race headset for a project and very much like the concept. As a DIY bike mechanic, the split race means that I can do clean headset replacements without procuring expensive tools or resorting to DIY hacks for the purpose. And, for headsets like the Velo Orange one that are running cartridge bearings rather than loose ball bearings, the cartridge assembly itself just bridges the gap. Unless I discover a good reason to not use split race headsets, I'll likely use them on as many of my bikes as I can in the future. In addition to the easy install, removal should also be easy. And for new forks, the crown race installation won't rough up the fork crown at all. I like the notion that I could pop headsets on and off of a bike without much fuss or having to sweat tolerances.

The split ring is typically aluminum and the steerer and crown race typically steel. I don't think the split ring will wear or damage the crown race.

I can't remember a steel fork I have serviced that used a split crown race. But there's plenty of carbon/carbon/carbon forks/crowns/steerers that have used a split race in my past. Andy