I like Smashwords – but uploading the MS is a bit of a pain. The meatgrinder as it’s known is notoriously fickle. On the plus side, it will throw the MS back and tell you what to fix. It can take several attempts before it goes through. The help pages on Smashwords are good and will offer advice.
One of the benefits of SW is the Premium Catalog https://www.smashwords.com/dashboard/channelManager/
You can submit your book, and have it distributed to a multitude of other sites – including Barnes and Noble, Kobo, I-books and many others. The most useful aspect I have found for Smashy is the coupons. You can produce a coupon to reduce a specific book, for a specific time. It’s great for gifts, review copies etc. Smashwords pay monthly (sort of). But the distribution stores pay at different times so it’s a little fiddly to keep track. That said it all goes through Smashwords and they pay via Paypal in USD. Or you can just stick with SW.
I have only added the pic for the first bit (as it’s quite long), but pretty self-explanatory.
You can also have a publisher account with SW. So, if you write under a pen name or publish on behalf of others then that works out nicely. It’s far more awkward on KDP – where you can publish under a pen name. The publisher account is helpful.
The dashboard for SW is reasonably easy to fathom and it’s easier to make changes to a book than on KDP and it’s better for readers as it offers Mobi, Epub and other formats (Amazon only offers the Amazon Mobi and it’s Kindle/Kindle app only).
Smashwords requires an ISBN but will provide one free if you don’t have one. This is required for access into the premium catalog, but not solely publishing on SW.
If you can manage the meatgrinder then Smashwords is a great way to get that wider reach.
It’s more accessible than KDP (see the other posts about this).