The problem comes when, as in this case, the accusers are referred to as victims without proof (apparently the police thought that to do otherwise would harm trust in the authorities but no-one qualified agrees with them and nor do actual victims - they expect to be regarded as complainants and will, whatever the police think, be referred to as such in court until the accused is convicted) and the police take the view that it is up to the accused to prove their innocence because they believe the accuser. A proper investigation has to start with an open mind and look at everything.
Clearly you must not only investigate the victim's believability, but failing to carry out any checks means you are reversing the presumption of innocence, turning it into an assumption that the accused is guilty. See, for example, the case of David Bryant who was wrongly convicted of an historic sexual assault by a complainant who alleged that this had deprived him of a promising boxing career. The truth emerged when the complainant tried to get damages from Bryant through the civil courts. The police had not investigated the alleged boxing career, nor had they looked at the complainant's medical records. They had believed him without question. Had they conducted these basic checks they would have discovered that there was no promising boxing career and the whole thing was a fabrication.
To put it another way, I broadly agree with the recommendations of the report by Sir Richard Henriques which you can find at factuk.org/wp-content/uploads/2017/04/Report-Independent-Review-of-the-Metropolitan-Police-Services-handling-of-non-recent-sexual-offence-investigations-1-3-1.pdf