Sheering can do a lot of harm to a sheep if one isn't careful. But so can cutting human hair. Have you seen Sweeney Todd?!?! I've had some very nasty wounds from having my hair cut.
There are steps to take to make it safe for the sheep. I cannot see anyone deliberately not taking these steps as harming a sheep means loss of money - one doesn't deliberately drive their car into a brick wall.
Per sheep, accidents are less likely to happen in large scale farming because
1. everyone there is experienced at working in these conditions
2. the sheep are sorted by colour, size, sex, health, etc. before sheering
3. the Shepheard knows what steps to take to keep the sheep as safe as possible
4. a sloppy sheering risks damage to the animal, loss of value to the wool (skin in the wool dries as hard as steel and can destroy expensive processing equipment), and loss of reputation (there aren't many shearers out there, and the teams are hired and paid by reputation as much as anything)
Accidents do happen, but considering how many hundred sheep are shorn per day, it is low and usually caused by other issues.
A gash in the side of the sheep looks nasty, but they have three excellent things going on to make it better than it looks.
1. being shorn kicks their metabolism into high gear so healing is fast
2. lanolin is antimicrobial and hardens into a crust after sheering to protect the sheep
3. they don't have as many nerve endings in their skin as we do (otherwise they wouldn't be able to tolerate the weight of the wool)
The big problem is with smaller farms the shearers may not be experienced or well trained. The farmer may not know how important it is to withhold food and
water for 12 or more hours prior to sheering and they may not know how harmful it is to shear wet sheep. Harm can come to the sheep out of ignorance and in my
experience, is more likely to happen in a small, hobby flock, than in an industrial setting.
I also want to point out, there are some videos of industrial sheering injuries going around. I met a person who was there and they told me that the person who did the damage to the sheep was paid quite a lot of money to do it for the
video. This is second-hand knowledge; I wasn't there. But from my experience with shearers - they are generally gentle, kind people who enjoy working with animals.