A few months ago, I moved my 45 titles of short stories, novellas, poetry and song lyrics from Smashwords and Amazon to Draft2Digital. I was impressed by D2D's slickness, which made the other sites look dated.
Uploading a manuscript to D2D takes seconds, unlike Smashwords where you sometimes join a queue to go through their
'meatgrinder.' Put it this way, it took me longer to unpublish my eBook on Smashwords than it did to publish them on D2D. I can republish my titles on Smashwords, should I choose to, as they're still there, just not browsable. D2D distributes to Amazon's KDP, so my books reappeared there. It also distributes to eBook vendors that I've never even heard of!
One thing to bear in mind, if you're UK based, is completing a US tax exemption form, otherwise most of your earnings will be taken by the American IRS. This is easy to do on D2D and Smashwords via an online process, but from memory it was trickier with Amazon.
No digital publisher or aggregator has magic bullets that guarantee you'll hit lots of readers. It's crucial to take into account self-promotion, to get your name and sparkling personality out there. Readers of books are not just after a tale to wile away a few hours—they want access to you and your life as an author—hence, the importance of having a blog (with a newsletter, so you can tell subscribers about your latest book), a website and making social media posts that are linked to those sites.
This is what I've been implementing this year, playing catchup on what I should have done years ago. I'm not a happy bunny...perhaps the Foreign Legion would take me...or, I could become a monk!