I think writing a story as a journal, or diary, can be somewhat liberating and fun actually. I wrote my first book (my only book) as a journal. I loved it. I actually had a great deal of fun, as I had to become the journalist; I was writing the journal as my main character. I became and experienced everything he did and wrote it to, and for my, fictional self.
I actually felt the emotional pain, and the loneliness, as well as falling in love. I 'felt' the cold breeze at the beach as I thought of what a jerk I was. It was great. In preparation I read other diariess: The Diary of Anne Frank, Flowers for Algernon, The Diary Of A Superfluous Man, and Dracula. I read them once then came back to them for ideas and counsel.
The only difficulty was that this story was a period piece — mid 1980s. So I had to verify that this Saturday was actually August 10th, and was this VHS tape actually out at that time, were those types of clothes in fashion then? A lot to juggle. It took a lot of time but I had fun nonetheless.
In general, I don't think there are any real upsides or downsides to writing anything. Just write your heart out. Love what you're doing, what you're crafting.
You're the main character, chronicling your thoughts and feelings. Expressing how you felt when you said something to somebody and what you thought about when they replied, and what you plan to do about it - or whatever. I do encourage you to read some or all of The Diary of Anne Frank; she writes to an imaginary friend. If you'd like a could send you my eBook, or download it for a buck on Amazon, and see how I handled it. It's called "Endings" by me, Roger Maxwell. Or again let me know what format you'd like and I'll email a free copy :0) firstname.lastname@example.org. I hope this helps.