The first big turnoff is that IngramSpark does basically the same thing as Amazon but charges you a fee. Not cool. Well, it would be cool if they offered customer support as a part of that fee, but apparently they take your money and tell you to figure it out yourself. Not only do the forums support this idea, but I have been through the process second hand. Something tells me if they offered even decent customer support they would help people get their stuff uploaded and these people would not have to talk to me.
Second--Amazon is the biggest book retailer in the world controlling almost a quarter of the field. If you publish through them, your book is automatically listed on their website and will come up in search results. If you publish through IngramSpark alone, someone will have to order your book through Amazon before it starts to become available there. You can get your book on Barnes and Nobles using IngramSpark, but again it will only be available through special order on their website. You are not going to get your book in a Barnes and Nobles store as an Indie writer unless you have a lot of really good connections and know-how. Just trust me--I have been in contact with my local B&N and I have truly thought about it, but if I realize the struggle and have decided that, no, that is not worth it, then you, too, should accept this fact. So, in short, IngramSpark is selling a pipe dream. Your book will not appear in-stock on the shelves of a major retailer simply by choosing IngramSpark.
IngramSpark (after paying your initial setup fee) requires you to register your ISBN yourself (which costs money). Once you do this, you need to set that ISBN up through Amazon if you are publishing both places, but then you lose Amazon's extended distribution option. I am not a fan of indie authors buying their ISBNs and trying to present themselves as an actually publishing business.
Now, you can get a hardcover from IngramSpark--but their pricing means that you will have to charge a whole lot more for your book than what is competitive. A higher price means more money in your pocket per sale, but it also means fewer sales--maybe 0 sales if your book is too expensive. Granted, CreateSpace stopped their hardcover service a couple years before the Amazon move. I hope they will bring this service back, but I am looking for other options. IngramSpark is not something I would consider putting myself through at this point. I am currently leaning toward B&N.
The biggest problem with IngramSpark is that it is not intuitive. People who have been doing it still do not understand everything they are doing and still end up having a lot of problems. So, no, I don't understand the novelty of paying for a headache. You should be able to upload a PDF to Amazon and then upload that same PDF to IngramSpark--many times you can't and that is a problem with the programming not with the user.
Lulu (expensive) and Smashwords (only e-books) were also rages at one point in time. It seems these are dying off, but two major print-on-demand venues have stayed with it: Amazon and B&N. Considering the options these offer for free, it boggles my mind people would choose to pay someone to distribute their books. Granted, considering the ease of uploading to Amazon, it boggles me why anyone would choose a difficult to navigate platform that is quirky in what it accepts and what it doesn't. As a self-publisher, you need to save money where you can and spend it on important things like better editing, better cover design, better formatting, and better marketing. Feel free to leave me a comment about the reason you chose (or avoided) IngramSpark. I don't usually allow links in comments, but if you link one book as an example I will allow it on this post only.