A novel in the form of a diary tends to concentrate attention on the narrator's PoV, even more than a more usual first-person narrative. It also tends to focus things more on the passage of tiem, because each entry must be made without knowledge of future events, while a non-diary first-person work can include intentional foreshadowing by the narrator, and retrospective comments.
Some readers will dislike the form, or find it confusing because it is no longer common (it was once a very common form). Others may enjoy it as a novelty. It avoids the classic problem in a first-person story of "when and why was this account written by the narrator?" A diary also lends itself to an unreliable narrator.
It is possible to have only part of the test be a diary, or to have multiple diaries, which allows for multiple POVs.
I would like to call attention to a particularly interesting and complex example: Freedom & Necessity by Steven Brust and Emma Bull (1997) It won the 1998 Locus award for "Best Fantasy Novel", although some would argue that it is not a work of fantasy but o historical fiction. It consists of fictional letters and diary entries, plus a few excerpts from manuscripts dates decades before the main action.
There are four main narrative points of view, as well as quit a few occasional writers. Each item is dated, and is from one person, and (if a letter) addressed to another. The reader must keep this in mind, be cause narrators will admit some things to some recipients, but conceal them (or even lie about them) from others. Also, letters cross in transit, so some writers are not aware of previously mentioned events, even those mentioned in a previous letter to that writer. So the reader must pay attention to who is writing to whom, and when. Goodreads thought well of it, Kirkus was less positive. The novel is set in 1849, mostly in England. Frederich Engels is a major but non-protagonist character, Marx has a couple of brief appearances. The outer facts of history are accurate. The hidden conspiracys are, I belive, fictional.