The best thing is that there’s no time pressure from a publisher or agent and you have complete control over your book, from its content to presentation. You can make more from the sale of each book and don’t have any commission to pay.
The worst side is having to do everything yourself, particularly the promotion, which can all end up costing quite a lot if you’re not careful. It also means you have less time to actually write and be creative, which can be frustrating when you have to drop everything to do administrative ‘stuff’.
I had an agent for a while who took my book to London and Frankfurt International Book fairs. The feedback I received was that publishers were only interested in new authors who had established their own promotion routes already, either with a large social media following or who are famous in one way or another. So if we have to do our own promotion, then the only advantage of having a traditional publisher is that they do all the work of preparing the manuscript and getting it into print. However, once you’ve done that a few times it does get easier