I do wonder if the Ussan Sangheili will ever matter in the current Haloverse. Broken Circle was just "ehh" for me though.
My three issues with Broken Circle were:
1. A lot of key plot points in the games and books that were big reveals were almost casually mentioned as if it were no big deal. A little bit of coyness can go a long way.
2. The pacing was rather hurried between events. The transition between the two time periods is rather jarring, and the conflict resolution was basically "Oh, I guess things worked out then. Ok..."
3. Every main character felt like a "progressive" version of the typical members of their species. The San'Shyuum like to get exercise and aren't scheming for their own agenda; the Sangheili men consider women as equals, and themselves are intelligent and not just bloodthirsty. I get it was supposed to make them likeable, but it felt cliche by the end.