No employee of mine would ever complain
Employers value their employees and appreciate the commitment and loyalty they see. This does not mean, however, that a loyal employee will not initiate an action with the DOL if their retirement appears to be in jeopardy. From the employee's perspective you and the company have an obligation to provide a reasonably vetted retirement platform to deliver "solid" investments at reasonable cost with relatively good service. If you fail to properly select your retirement plan service providers and properly supervise them, the employee is certainly within his rights to question you and potentially to file a complaint.
The Department of Labor initiates random audits without the impetus of a complaint from a Plan participant.
The IRS can audit your plan and refer any findings of interest to the DOL for further attention.
The IRS burden for reviewing GUST Restatements of Plan documents is largely behind it, allowing a significant number of Plan Reviewers to be trained as Field Auditors. Practitioners expect the frequency of IRS audits to rise in coming years.