I loaded the fixer theme today as a starting point for a new site I'm setting up (darkhollowinternet.com). I saw several issues that all seem to stem from fixer/upfront requiring rewrites be enabled. Due to historical issues, I've kept them off so it caused two issues for me:
1) Homepage failed to display background image
2) When trying to change background image elements in editor, it doesn't load properly and shows the dialog at the bottom of the page with poor styling.
Both of these issues appear to stem from the use of the /upfront-dependancies fake directory (url is http://darkhollowinternet.com/upfront-dependencies/styles/d25703bb5ae070246833b909ebe61d60?ver=0.1.2).
It appears that this is similar to the issues seen by kendavis in https://premium.wpmudev.org/forums/topic/problems-with-upfront-and-spirit-template . The solution of adding AllowOverride All is a bit excessive and not something that i want to use. AllowOverwrite FileInfo allows the redirectengine to work which solved the problem temporarily but I would prefer to not require usage of the .htaccess file or allow any overrides in the web directories.