But ... Vellum's pretty inexpensive in the grand scheme of things and remarkably easy to use, so I'm now convinced there's an ROI there. I'm still uncertain whether most readers will notice a difference, but I think it's worth it for those who will.
For me, the value has definitely been in the time spent/hair pulled that I used to have. My process was pretty basic to start with. Write in Scrivener, compile to word doc for editor, import final word doc back into Scrivener, compile to epub and tweak in Sigil. I'd make unique backmatter for different vendors so that was a bit fiddly.
No I take the final word doc, import to Vellum, copy the backmatter from the previous book as the product IDs carry over, select the options for formatting and generate the files. As far as I'm concerned it's already paid for itself by saving my sanity.