Sorry to say this, but this has been our experience too.
And no, it's not that dnephew is racist and only 'plays up' for some carers. That was actually implied by one agency. Most of his carers are diverse in race, and generally from the UK or Poland/Romania. They've always taken care of him like one of their own, very kind and treat him as a person who can understand them. My sister has never had a cpmplaint about them for years.
Nephew has very high needs, is non verbal and, for the want of a better word, drools.
One new carer left bruises all up his arm, she was that rough. The others have been to a one, rude to nephew and sister, very dimissive and refuse to even attempt to talk through things with dnephew before they do them, only talking to each other. They treat him like an inanimate object they have to clean and then get out as quickly as possible.
Sister has had to complain repeatedly and is very distressed about what will happen when she isn't there to do that anymore.
I also have to wonder, surely if there was shortage of care workers, wouldn't that be better? Wages would be forced to go up and it could be paid the way it had always deserved. It is often a job that women who have been out of work get after their children are older.