Howard Gleckman highlights a potential problem with 401(k) plans that automatically enroll employees:

Mauricio and Barbara found that employer match rates are about 7 percentage points lower for opt-out plans. They can’t say for sure whether auto enrollment causes lower match rates. But it sure is possible. After all, if more employees participate, their employers will have to spend more to match their contributions. Whatever the cause, it seems that while auto-enrollment may increase the number of workers with 401(k)s, it won't necessarily boost their retirement savings.