One of my biggest problems is that what you say is true in the way you mean it but it isn't really true. The way an employer gets at it is passing along all plan costs to the employees who then have their accounts debited every quarter for those costs. I think making employees pay for the benefit really lessens its effectiveness as a benefit (particularly if the plan has no matching).

Another way this is a problem is that the fees are typically debited from the employees based on their balance in an account. So if a 20 person company has five employees with very large balances and fifteen employees with very small balances then the five employees with the larger balances are subsidizing the other fifteen. You end up penalizing the people who have been the most dedicated to saving for retirement.

It seems like Guideline and OctaveWealth (mentioned in the comments) combat this by charging a recordkeeper-style per-participant fee rather than the percentage of assets that are typically charged by advisors and custodians.