I've done semen tests using an electroejaculator, and when collecting a bull for BSE, if the first go is not good, you have to do another two before you can declare him a dud. The males (vets) I worked with would go high power, get a quick response and then couldn't get the second or third as needed. I started off low, slowly increased the power and rarely got anywhere near their levels--and would be able to collect several times. Extrapolating from that, if done often at high levels, nerve damage is possible. BUT, this is no indication that the damage is lasting.
The fact is, prostatic massage isn't always successful, you don't always have the ability to tease and use an AV, and training a bull to mount a steer and breed the AV isn't something you're going to do for anything but (AI )collection anyway. They still use the electroejaculator at the vet colleges to BSE client's animals--and these guys write the books. I'd like to see data, not just heresay, against the practice before I'd stop doing BSEs.