IMHO, the current maintainer
user_none does a great job adding new features to Sigil all the time and quickly addresses critical bugs. While there are some minor bugs and quirks, Sigil is easily on par with commercial programs and most people who complain about robustness issues or lack of user friendliness often simply lack the necessary skills to properly use Sigil, and many of them would probably be better off using other programs.
BTW, since Sigil is
CC BY-SA 3.0 licensed, theoretically anybody could grab the code and create a commercial derivative under a different name as long as they abide by the Creative Commons rules.