From my understanding, Hathora is a netcode, so the layer handling communications between players through the server. Agones is for orchestrating the deployment/scaling of game servers, and Open Match is for matching players together, who will then join this server. So they are all complementary. Being in the industry, not a fan of Agones, and I barely hear of anyone using it. It solves the wrong problem, it's supposed to be a solution for game studios to handle scaling, but the only studios that have the resources to spend to understand, use and manage it already have their own solution in place. Open Match on the other hand is a great tool for matchmaking.

I am familiar with Agones and I had briefly looked into leveraging Open-Match as a matchmaking option for Hathora.

To me, part of the value Hathora adds is from being an integrated and easy to use solution where you don't need to figure out how to put the parts together in order to develop your game.