He's accused Luis Paulo Supi (Brazillian grandmaster) a few times after losing to him, and he accused Andrew Tang after losing to him. The latter was criticized in some online circles because it was seen as bullying a then 14 year old.
I don't know many other notable cases of Nakamura accusing players of cheating. Many players dislike how Nakamura conducts himself on stream and how he interacts with the chess community and this leads to exaggeration. It's simply wrong to compare him to Kramnik, who has dedicated many hours over the last couple years to accusing players.
Hikaru accused Luis Paulo Supi of cheating at least twice.
From his Wikipedia article:
```
In an online blitz tournament hosted by the Internet Chess Club in May 2015, American Grandmaster Hikaru Nakamura accused Supi of cheating (Supi had defeated Nakamura).[2] The tournament judges accepted Nakamura's accusation, reverted the match's result, and banned Supi from the tournament. Brazilian Grandmaster Rafael Leitão wrote in his personal website, "Accusing him of using an engine in this match is absurd. The match is full of tactical mistakes. Nakamura played extremely poorly and, honestly, wouldn't have survived long against any engine given his terrible opening.".
```
Some years later Nakamura lost 4-0 and again insinuated that GM Supi used an engine.
Despite all that, Nakamura still published a video calling him a "legend" for once beating Magnus in 18 moves
