I think there is absolutely no incentive for a recruiting service to unfairly rank a kid lower than what their ability reflects. Recruiting services stake their reputation on being accurate and correct so there’s no incentive to do what many assert them doing.
Also, their accuracy is reflected in their class ranking and comparing it to end of the year results for college football teams. In other words, look at who finishes in the top 10 each year and correlate that with how they’ve been recruiting based on where their classes have ranked. Sure there are exceptions, but more times than not, the recruiting services get it right.