At the risk of putting in my opinion where others, better qualified, have expressed theirs I think the clue is in the name - Book VIEW. It's fine for viewing what you are going to see and does that job well; the problem comes if you try to use it for editing.
I used Sigil for quite a time before I discovered you could use Book View to edit. It wasn't long before I saw it wasn't a good idea.
Personally I tend to use Calibre's editor for most things but there are some things that Sigil does that Calibre doesn't and then I use Sigil for that.
The important thing is to know your tools and to use the most appropriate one for the job in hand.
BobC
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