Quite true. An investigation, demanded by state representative Jose Canales, into Ranger abuse against Mexican Americans resulted in the reduction of the Ranger force. What was considered stricter qualifications for Ranger recruits was also instituted.
The investigation, with all its gory details, produced over a thousand pages of testimony . . . all of which has been available to the public since the day it was published.
The Rangers also get blamed for abuses they did not commit. Dusing the 1917-1919 border scares another organization, the Loyalty Rangers, certainly instilled fear among some Texans. Anyone from any ethnic group with perceived questionable allegiance might have their actions looked at. But, Loyalty Rangers, created by the Hobby Loyalty Act, were not Texas Rangers.
None of this was ever "covered up" that I remember. We were taught much of in Texas History classes.