It is viewed as "more risky" not because it has been proven as risky, but because there is less data on it.
Artificial Disc Vs. Anterior Cervical Discectomy and Fusion
"ACDF has been around decades longer than today’s artificial cervical disc options. While abundant data is available regarding the long-term safety and efficacy of ACDF, similar data is not yet available for cervical ADR. As a newer technology, one of the main risks of cervical ADR is not knowing how long the artificial disc will last, even though results thus far have been promising."
The entire argument is "it's newer, so less is known". That doesn't make it inherently more risky.