No Optical Stabilization.
I don't see why anyone would buy it over the nearly perfect IQ of the 135L. The 135L is probably top 3 Canon sharpest lenses and mine is nearly as sharp at F/2 as it is at F/5.6. I can only assume the 135 ART will be priced $1200+ and without OS. 1/3 of a stop more light won't justify the cost of this lens over the 135L.
With all that said, I picked up my 135L from local Craigslist in perfect condition for $650.
Because it will be sharper and has F/1.8. Why would you buy the Canon? It's 20+ years old!
Canon don't seem to innovate much any more. Thirty years ago they innovated with EF mount, and a slew of faster than ever AF lenses (50/1.0, 85/1.2, 200/1.8), then they did TS, and they owned the pro low-noise digi market with the EOS-1D Mark II.
Fast forward 30 years and a 3rd-party manufacturer is teaching them a thing or two about innovation.