No customization presents a simple tool that, hopefully, just works and can be accepted with its flaws and limitations. Once customization starts being added in, it's easier to see annoyances and "why isn't this feature present".
Not really. Generally, ALL CAPS and italic are preferred over bold (as both are specially cut for that, and keep the contrast roughly the same), but bold is not bad either (and for sans serif, always choose bold over italic). Compare with: http://practicaltypography.com/bold-or-italic.html
I am not quite sure about it not being good typographic practice, but it's sure weird historically to mix faces of different weights. Furthermore there is already emphasized (italic) text; it's not strictly necessary for prose.
I've read a handful of books where there is some type of bold formatting used, and in each case it was electronic rich text (e.g an email or word document).
I've just imported a story I'm editing into Quoll. It seems quite nice. Little visual clutter, a decent feature set that helps with keeping track of characters, scenes etc, organize ideas, keep track of word count targets, keep you motivated etc. The achievements are a nice touch, and the ability to invite editors looks helpful (though I haven't tried that)
There are a few bugs (it reordered my chapters when I marked a chapter as edit-complete the first time. That obviously shouldn't happen).
Overall it seems like a big step up compared to Word. It can't quite keep up with the feature set of scrivener, but in exchange it has a much more approachable and clutter-free interface, much closer to the idea of distraction-free writing.
I would say the export functions are among the weaknesses of Quoll. You can export to HTML, docx and epub, with the only notable configurable option the selection of the chapters (and character notes etc) that should be included in the export. For epubs you can also configure author name and ISBN.
The quality of the exports is not that great. The HTML looks good for printing, but is a pain to read on screen. The epub looks good, except that it doesn't display chapter headings. The docx has 8 newlines between paragraphs when I open it in OpenOffice. I don't have a MS Word on hand right now to check whose fault that is, but it's unfortunate since I would probably go via docx to pdf.