Although the Acas guidelines are not actually mandatory the failure to follow them can result in compensation being increased by 25%.
One of the more difficult areas impacting on grievance investigations is complaints of bullying and harassment. The ACAS guidance on bullying and harassment deals with the possibility of informal resolution to complaints by employees about being subjected to bullying and harassment. Whilst that ultimately is good advice from the point of view of employee relations, the difficulty facing an employer is knowing when a grievance has been lodged or not. A complaint about bullying could easily be regarded as a formal grievance. The lack of formal response will leave the employer exposed to the risk of claims for constructive dismissal.
Remember to pay heed to your own policies on Discipline & Grievance as a Tribunal is likely to say that any investigation has to comply with whichever of the Acas code and your policies is the most prescriptive of what should be applied to the dismissal. It is potentially damaging just to adopt a Trade Association or copy and paste from the Internet without giving some thought to what you are implementing
There may also be occasions when an employer needs an independent organisation to actually handle a disciplinary investigation or grievance. This may be because it involves a high level employee or the subject matter is sensitive, such as discrimination or harassment.