Ariane 5 rockets have accumulated 109 launches since 1996, 104 of which were successful, yielding a 95.4% success rate. Between April 2003 and December 2017, Ariane 5 flew 82 consecutive missions without failure, but the rocket suffered a partial failure in January 2018
There are several big risks. It's impossible to rank them in a credible way.
Obviously there are all the usual launch risks. The cryogenics system has had a lot of development problems. The deployment is staggeringly complex and involves components that are not known for their robustness. Finally there is no repair option; one critical things goes wrong or is found to be misdesigned and that's it.
