Risks are mostly it not working, or bleeding. Bleeding is likely to be minimal, since this is basically as invasive as getting a shot, or rather about forty shots, all near a blood vessel. Bruising is likely in the sperm cord.
In my experience, marcaine nerve blocks work better than lidocaine.
So kenelog is a potent steroid, and anti-inflammatory.
A nerve block is either a treatment or a test. If it's a test, you're going to need to move on to castration, or microsurgical denervation of the sperm cord. If it's a treatment, you need a therapeutic mechanism. Fortunately, a long-lasting anti-inflammatory is a good option. This works on the basic principle that "nerves that fire together wire together" -- by interrupting the firing with painkillers and anti-inflammatories, nerves stop firing together, and thus have a chance to heal.
Kenelog is a powerful steroid. Novocaine works fast, and blocks all pain in the region. Marcaine works a little slower - but can last for two days, plenty of time for the kenelog to make it stop hurting in the long term. We do this because of the "rough morning" phenomenon. Novocaine didn't keep me numb long enough for the injections to quit hurting!
Marcaine with kenelog, however, in a ratio of 9:1 (in the syringe) did the job. It masked the pain of the injection, and allowed the nerve pain to subside.
Inflammation in a membrane bound space like the testicle or sperm cord can damage the testicle, long after the cause of the inflammation is gone. Swelling inside a "tissue baggie" like that is indistinguishable from squeezing the contents! It friggin' hurts! With the kenelog to interrupt the swelling, the pain can stop, and the injured tissue that's releasing inflammatory cytokines can heal.