Sigil is great for polishing ebooks and correcting errors, removing unused styles and images, creating a table of contents when there is none, removing useless portions of code, correcting misspellings. (Thanks to ebooks, now, when we find a misspelling in a book, we are no longer condemned to read it again and again till the end - which is bad for my blood pressure, as I'm a former French teacher - we can fix it
Calibre is the most essential tool for e-books, but it is not the best epup creating tool. Amanuensis and Writer2epub, and even the writer2xhtml addon for LibreOffice create cleaner output files.
But maybe my poor opinion about Calibre for making ebooks can also be explained because it is mostly used by noobs who convert books without understanding what they are doing.
Every time I made a book from an ODT with Calibre, it proved to be good, except for some too complicated stylesheets. But if you avoid direct formatting and use predefined word processor stylesheets for everything except bold and italic within a paragraph, Calibre, Amanuensis, writer2epub and Atlantis make excellent and light epubs.