Ascensus is also reporting that adoption of guided qualified solutions, such as target date funds, continued to spike throughout its client base. In 2011, a little over 17 of assets in the Ascensus universe were invested in model portfolios or TDFs. By 2014 the total jumped to 26 percent.

That data jibes with industry-wide reports showing increasing auto-enrollment and TDF usage.

But recent reports have called into question both automatic enrollment's and TDF's effectiveness in assuring participants are adequately preparing for retirement.

Wells Fargo reported that participating eligible employees rose 13 percent between 2011 and 2015, as 40 percent of Wells-administered plans now deploy an auto-enrollment feature, up from 30 percent in 2011.

But deferral rates remained flat in that period. About 38 percent of all participants defer at least 10 percent of their salary in Wells plans, a slight increase from 34 percent four years ago: 28 percent of millennials contribute at least that much, compared to 35 percent of Gen Xers and 45 percent of baby boomers.

And a recent study from the Center for Retirement Research at Boston College found that older workers don't tend to contribute as much to the plan as workers who have to actively put themselves into the plan.

Those figures don't discount the necessity of auto-enrollment going forward, said Geno Cufone, senior vice president for retirement administration at Ascensus.

"While auto-enrollment coupled with auto increase is the best way to help employees save for retirement and gives them the best chance to be financially secure during retirement, our data suggests that adopting auto-enrollment without auto increase is still better than adopting neither," said Cufone.

As a stand-alone feature, auto-enrollment is particularly valuable for younger employees, added Cufone.

The standard 3 percent default deferral rate is still the most popular with Ascensus' clients, but employers are adopting higher rates 18 percent of the time, with some adopting an initial deferral rate of 10 percent, according to Cufone.

From his vantage at Ascensus, sponsor engagement is as high as it has ever been.

"We're seeing a record number of small employers start new retirement plans, more sponsors are adding or increasing employer matching contributions, and they're adopting automatic features at a rate that has steadily increased on a year-over-year basis."

All signs that sponsors view retirement plans as necessary to attract and retain talent, he said.

"Sponsors' actions suggest that they understand they need to do more than just offer a retirement plan. The vast majority of them partner with a financial professional to help educate their employees and make the best choices to increase their opportunity for financial independence during retirement."

Recently, more reports from regulators and stakeholders are suggesting the widespread adoption of TDFs could be putting some savers at risk.

One study from Northern Trust showed that as more assets are channeled into TDFs, a disproportionate amount is going into U.S. equity funds.

And earlier this year, Securities and Exchange Commissioner Luis Aguilar called on the agency to do more to warn anyone saving for retirement about the "inherent risks" in target-date funds.

He called the "relentless" growth in TDFs "troubling" in a speech at the American Retirement Initiative's winter summit, and he cited an SEC study that showed only one-third of respondents could correctly identify what the date in "target date" meant, and only 36 percent understood that they do not guarantee income.

Moreover, TDFs have been increasing their equity exposure since 2005, said Aguilar. Other studies suggest the majority of advisors mistakenly assume TDFs are more conservatively invested than they actually are.

"The perception of these funds being safe is further fueled by most retirement plans utilizing them as their default investment," which implies they are among the safer options available to 401(k) investors, warned Aguilar.

The degree to which sponsors share Aguilar's concerns has not necessarily been established.

At Ascensus, Cufone reports that he isn't seeing any widespread concern over TDFs from plan sponsors.

Overall, TDFs now account for 12 percent of assets in Ascensus' platform, and once sponsors go with a TDF option, they tend to stick with it, as less than 1 percent of sponsors have eliminated TDFs from lineups once to offering is established, he said.