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the halo threadless cogs work very well wih their hubs, they have a footplate to spread stress along the splines. if you just put a standard cog from a cassette on (it fits, as the splines are the hg pattern) then yes, it will tear through the splines, as all that fixie power braking force will be going through a 2mm or less interface.
there's alot to be said for using bike parts as they're designed to be used.
jakjakjak-
I understand the larger footprint aspect of the Fat Foot Halo Cogs, But really they need to be an interference fit on the spines on the hubs, If you load up the cog accelerating, and use it to brake too , the cog will move back and forth on the splines, only a tiny amount to start with, but with the two different harnesses of material, the steel cog will wear the aluminum hub away reasonably quickly. I use Fat Foot Halo Cogs on a Hope Hub on my singlespeed winter bike, Great for that, the greater footprint doesn't damage the freehub body like the old modified sturmey archer sprockets I used to use
Keppel
I think the if you want to go for threadless cogs, Halo isn't the way to go, I've heard stories of the hubs failing due to Steel sprocket fretting the aluminium hub body.
The Level cog system was awesome, not sure if its still around, the White Industries version is nice too.
I have a 6 bolt ISO setup with a Velo Solo cog onto Mack hubs. Its been on my bike for over 5 years without having to touch it.
Not a massive fan of regular threads/lockrings myself.