Curiosity, it’s not a comparison. Rebbe Nachman’s explicit advice to his followers was to visit his kever on Rosh HaShana. Breslovers are doing just that — following his advice. If you don’t like the word advice, well then call it a custom, as Rebbe Nachman does himself (see below). Surely it’s legitimate to follow a minhag established by your rebbe?
“It is customary to go to the Tzaddik to celebrate Rosh Hashanah. Rosh Hashanah is first and foremost the Day of Judgement. But no matter how great the severity of the judgement and no matter where in the world it threatens, everything is sweetened when Jews gather around the Tzaddik on Rosh Hashanah. With so many Jewish souls gathered together and merged in abundant love, a wonderful joy and delight come into the world.”(From Likutei Eitzos, Rosh HaShana).
Also, from Chayey Moharan (#403):
“Everyone, without exception, who counts himself as one of my followers or takes heed of what I say should come to me for Rosh Hashanah. Anyone who is worthy of being with me for Rosh Hashanah should be very happy: ‘Go your way, eat the fat and drink the sweet, for the joy of the Lord is your strength’ (Nehemiah 8:10).
“The Rebbe once declared: ‘Gohr mein zach is Rosh Hashanah . . . My entire mission is Rosh Hashanah.’ He was particularly emphatic about his followers coming to him for Rosh Hashanah, and indicated on his last Rosh Hashanah in Uman that we should continue to do so even after his death.”