I had the code-bloat discussion one time with someone, I think on Kboards, but it might have been on KDP community forum (before the automated moderation drove me away for good). Yes, there's code bloat, but when I questioned the person about the
impact of code bloat, he wasn't sure there was any. Vellum doesn't seem to produce an overly large file, its book files don't trigger error messages, at least not that I've ever seen, and they don't seem to load more slowly than non-Vellum books. Sometimes, it doesn't display properly in the Look Inside without some nagging of support, but that's a function of Amazon not being consistent on its end, and the problem is not unique to Vellum.
In other words, I can see a purist wanting to avoid unnecessary code, but if it doesn't detrimentally affect the reader experience, I don't care.
A professional formatter could doubtless produce a better result. Vellum does what it does really well, but a lot of things can't be customized. However, if an author likes the look of a Vellum file, it is less expensive in the log run to use Vellum than it is to hire a formatter every single time. Vellum's current price for the ebook only version is $199; the version that includes paperback formatting is $249. I just checked the site of a formatter I'm aware does outstanding work and is known for integrity. Assuming I submit a clean Word file, a novel in my normal length range would be $225-$250--just for the MOBI and EPUB files. Files for paperback publishing are also available for an addition charge. The charge is not specified (The site says, "See our POD Package pricing plans," but I couldn't find them.)
In other words, hiring a good formatter could run as much, just for ebook formatting for one book, as Vellum runs for unlimited books, ebook and paperback. For an author who plans to write a lot, Vellum becomes more cost effective very quickly.
I used to do my own ebooks (Scrivener, with an occasional tweak in Sigil). I had no problem with that. But Scrivener doesn't work as well with PDFs for paperbacks, and it took me about a week to do the PDF formatting by hand. Vellum does it better and does it in about 30 seconds. Could I get a more elegant design from a pro? I'm sure. But for the number of paperbacks I sell, it's just not worth spending a lot of money to jump from good formatting to great formatting. I'm a voracious reader, but I buy books for content, not pretty formatting. As long as the formatting is easily readable and free from distracting inconsistencies or other problems, I'm fine.
There is one caveat--Vellum doesn't really support nonfiction features like tables, appendixes, indices, footnotes, etc. But for ordinary fiction, it would be more than enough for most authors.
It's also worth noting that its still only available for the Mac.
I had an old Macbook I'd used at work that could run Vellum. If an author had to buy a Mac just to use it, that would change the cost factors considerably.
It's also worth noting that, aside from Kindle Create, D2D also offers some basic formatting options (and seems to be gearing up to do paperback). I haven't used them for that, but some people have described them as a good free alternative.