Most are unionized but not all. Kroger's union is also just not very good. I never met a union rep that wasn't friends with management. They do very little to actually represent workers. Also, Kroger has such a high turnover rate that newer employees will usually vote to approve any contract that gives them a raise, no matter how small.

These contracts last for a few years, so as other retailers raise wages to compete with each other, Kroger workers are stuck in their old contract until it expires. If anything, Kroger's union results in lower wages for the workers compared to other grocery retailers.

UFCW is further weakened by "right-to-work" laws that give union benefits to non-union employees. In my experience, many of the people who opt out of union membership are brainwashed by conservatism and are anti-union. They are also often the people who stay at Kroger the longest.

In addition, Kroger has dozens of "districts", each with their own contract. These contracts are all negotiated at different times so that if there is a strike, Kroger can send managers from other districts and hire scabs until the strike is over. This prevents strikes from ever really hurting Kroger's bottom line.

So, to answer your question, Kroger is technically unionized but it's union is pretty ineffective. Kroger basically operates the same as Walmart or Meijer but with a handful of union policies that they have to follow or they might get fined for a union violation.

With that being said, I am still very pro union. The above is just based on my own experience, having worked there for too many years.